[5/5] drm/amdgpu: implement amdgpu_gem_prime_move_notify v2
diff mbox series

Message ID 20200217154509.2265-6-christian.koenig@amd.com
State New
Headers show
Series
  • [1/5] dma-buf: add dynamic DMA-buf handling v14
Related show

Commit Message

Christian König Feb. 17, 2020, 3:45 p.m. UTC
Implement the importer side of unpinned DMA-buf handling.

v2: update page tables immediately

Signed-off-by: Christian König <christian.koenig@amd.com>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 66 ++++++++++++++++++++-
 drivers/gpu/drm/amd/amdgpu/amdgpu_object.c  |  6 ++
 2 files changed, 71 insertions(+), 1 deletion(-)

Comments

Daniel Vetter Feb. 17, 2020, 5:55 p.m. UTC | #1
On Mon, Feb 17, 2020 at 04:45:09PM +0100, Christian König wrote:
> Implement the importer side of unpinned DMA-buf handling.
> 
> v2: update page tables immediately
> 
> Signed-off-by: Christian König <christian.koenig@amd.com>
> ---
>  drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 66 ++++++++++++++++++++-
>  drivers/gpu/drm/amd/amdgpu/amdgpu_object.c  |  6 ++
>  2 files changed, 71 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> index 770baba621b3..48de7624d49c 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> @@ -453,7 +453,71 @@ amdgpu_dma_buf_create_obj(struct drm_device *dev, struct dma_buf *dma_buf)
>  	return ERR_PTR(ret);
>  }
>  
> +/**
> + * amdgpu_dma_buf_move_notify - &attach.move_notify implementation
> + *
> + * @attach: the DMA-buf attachment
> + *
> + * Invalidate the DMA-buf attachment, making sure that the we re-create the
> + * mapping before the next use.
> + */
> +static void
> +amdgpu_dma_buf_move_notify(struct dma_buf_attachment *attach)
> +{
> +	struct drm_gem_object *obj = attach->importer_priv;
> +	struct ww_acquire_ctx *ticket = dma_resv_locking_ctx(obj->resv);
> +	struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
> +	struct amdgpu_device *adev = amdgpu_ttm_adev(bo->tbo.bdev);
> +	struct ttm_operation_ctx ctx = { false, false };
> +	struct ttm_placement placement = {};
> +	struct amdgpu_vm_bo_base *bo_base;
> +	int r;
> +
> +	if (bo->tbo.mem.mem_type == TTM_PL_SYSTEM)
> +		return;
> +
> +	r = ttm_bo_validate(&bo->tbo, &placement, &ctx);
> +	if (r) {
> +		DRM_ERROR("Failed to invalidate DMA-buf import (%d))\n", r);
> +		return;
> +	}
> +
> +	for (bo_base = bo->vm_bo; bo_base; bo_base = bo_base->next) {
> +		struct amdgpu_vm *vm = bo_base->vm;
> +		struct dma_resv *resv = vm->root.base.bo->tbo.base.resv;
> +
> +		if (ticket) {

Yeah so this is kinda why I've been a total pain about the exact semantics
of the move_notify hook. I think we should flat-out require that importers
_always_ have a ticket attach when they call this, and that they can cope
with additional locks being taken (i.e. full EDEADLCK) handling.

Simplest way to force that contract is to add a dummy 2nd ww_mutex lock to
the dma_resv object, which we then can take #ifdef
CONFIG_WW_MUTEX_SLOWPATH_DEBUG. Plus mabye a WARN_ON(!ticket).

Now the real disaster is how we handle deadlocks. Two issues:

- Ideally we'd keep any lock we've taken locked until the end, it helps
  needless backoffs. I've played around a bit with that but not even poc
  level, just an idea:

https://cgit.freedesktop.org/~danvet/drm/commit/?id=b1799c5a0f02df9e1bb08d27be37331255ab7582

  Idea is essentially to track a list of objects we had to lock as part of
  the ttm_bo_validate of the main object.

- Second one is if we get a EDEADLCK on one of these sublocks (like the
  one here). We need to pass that up the entire callchain, including a
  temporary reference (we have to drop locks to do the ww_mutex_lock_slow
  call), and need a custom callback to drop that temporary reference
  (since that's all driver specific, might even be internal ww_mutex and
  not anything remotely looking like a normal dma_buf). This probably
  needs the exec util helpers from ttm, but at the dma_resv level, so that
  we can do something like this:

struct dma_resv_ticket {
	struct ww_acquire_ctx base;

	/* can be set by anyone (including other drivers) that got hold of
	 * this ticket and had to acquire some new lock. This lock might
	 * protect anything, including driver-internal stuff, and isn't
	 * required to be a dma_buf or even just a dma_resv. */
	struct ww_mutex *contended_lock;

	/* callback which the driver (which might be a dma-buf exporter
	 * and not matching the driver that started this locking ticket)
	 * sets together with @contended_lock, for the main driver to drop
	 * when it calls dma_resv_unlock on the contended_lock. */
	void (drop_ref*)(struct ww_mutex *contended_lock);
};

This is all supremely nasty (also ttm_bo_validate would need to be
improved to handle these sublocks and random new objects that could force
a ww_mutex_lock_slow).

Plan B would be to throw our hands into and declare that "move_notify is
best effort only and can fail for any reason". Exactly like ttm eviction
currently does, even with all your hacks to do at least some dma_resv_lock
(but not the full slowpath).

Given how much "fun" you have with all the low memory handling and ttm
fundamentally being best-effort only (despite that dma_resv would allow us
to do this right, with some work) I'm not sure that's a good idea to
extend to a cross-driver interface. Personally I'd lean towards fixing
this first fully (in ttm/amdgpu), and then using that to implement
move_notify correctly.

Or just add an int return value here and mandate that importers must
handle eviction failures. Exactly like ttm_mem_evict_first can currently
still fail for various reasons.

(Sorry this isn't exactly the mail you hoped for)

Cheers, Daniel

> +			/* When we get an error here it means that somebody
> +			 * else is holding the VM lock and updating page tables
> +			 * So we can just continue here.
> +			 */
> +			r = dma_resv_lock(resv, ticket);
> +			if (r)
> +				continue;
> +
> +		} else {
> +			/* TODO: This is more problematic and we actually need
> +			 * to allow page tables updates without holding the
> +			 * lock.
> +			 */
> +			if (!dma_resv_trylock(resv))
> +				continue;
> +		}
> +
> +		r = amdgpu_vm_clear_freed(adev, vm, NULL);
> +		if (!r)
> +			r = amdgpu_vm_handle_moved(adev, vm);
> +
> +		if (r && r != -EBUSY)
> +			DRM_ERROR("Failed to invalidate VM page tables (%d))\n",
> +				  r);
> +
> +		dma_resv_unlock(resv);
> +	}
> +}
> +
>  static const struct dma_buf_attach_ops amdgpu_dma_buf_attach_ops = {
> +	.move_notify = amdgpu_dma_buf_move_notify
>  };
>  
>  /**
> @@ -489,7 +553,7 @@ struct drm_gem_object *amdgpu_gem_prime_import(struct drm_device *dev,
>  		return obj;
>  
>  	attach = dma_buf_dynamic_attach(dma_buf, dev->dev,
> -					&amdgpu_dma_buf_attach_ops, NULL);
> +					&amdgpu_dma_buf_attach_ops, obj);
>  	if (IS_ERR(attach)) {
>  		drm_gem_object_put(obj);
>  		return ERR_CAST(attach);
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
> index 8ae260822908..8c480c898b0d 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
> @@ -926,6 +926,9 @@ int amdgpu_bo_pin_restricted(struct amdgpu_bo *bo, u32 domain,
>  		return 0;
>  	}
>  
> +	if (bo->tbo.base.import_attach)
> +		dma_buf_pin(bo->tbo.base.import_attach);
> +
>  	bo->flags |= AMDGPU_GEM_CREATE_VRAM_CONTIGUOUS;
>  	/* force to pin into visible video ram */
>  	if (!(bo->flags & AMDGPU_GEM_CREATE_NO_CPU_ACCESS))
> @@ -1009,6 +1012,9 @@ int amdgpu_bo_unpin(struct amdgpu_bo *bo)
>  
>  	amdgpu_bo_subtract_pin_size(bo);
>  
> +	if (bo->tbo.base.import_attach)
> +		dma_buf_unpin(bo->tbo.base.import_attach);
> +
>  	for (i = 0; i < bo->placement.num_placement; i++) {
>  		bo->placements[i].lpfn = 0;
>  		bo->placements[i].flags &= ~TTM_PL_FLAG_NO_EVICT;
> -- 
> 2.17.1
>
Christian König Feb. 17, 2020, 6:58 p.m. UTC | #2
Am 17.02.20 um 18:55 schrieb Daniel Vetter:
> On Mon, Feb 17, 2020 at 04:45:09PM +0100, Christian König wrote:
>> Implement the importer side of unpinned DMA-buf handling.
>>
>> v2: update page tables immediately
>>
>> Signed-off-by: Christian König <christian.koenig@amd.com>
>> ---
>>   drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 66 ++++++++++++++++++++-
>>   drivers/gpu/drm/amd/amdgpu/amdgpu_object.c  |  6 ++
>>   2 files changed, 71 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>> index 770baba621b3..48de7624d49c 100644
>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>> @@ -453,7 +453,71 @@ amdgpu_dma_buf_create_obj(struct drm_device *dev, struct dma_buf *dma_buf)
>>   	return ERR_PTR(ret);
>>   }
>>   
>> +/**
>> + * amdgpu_dma_buf_move_notify - &attach.move_notify implementation
>> + *
>> + * @attach: the DMA-buf attachment
>> + *
>> + * Invalidate the DMA-buf attachment, making sure that the we re-create the
>> + * mapping before the next use.
>> + */
>> +static void
>> +amdgpu_dma_buf_move_notify(struct dma_buf_attachment *attach)
>> +{
>> +	struct drm_gem_object *obj = attach->importer_priv;
>> +	struct ww_acquire_ctx *ticket = dma_resv_locking_ctx(obj->resv);
>> +	struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
>> +	struct amdgpu_device *adev = amdgpu_ttm_adev(bo->tbo.bdev);
>> +	struct ttm_operation_ctx ctx = { false, false };
>> +	struct ttm_placement placement = {};
>> +	struct amdgpu_vm_bo_base *bo_base;
>> +	int r;
>> +
>> +	if (bo->tbo.mem.mem_type == TTM_PL_SYSTEM)
>> +		return;
>> +
>> +	r = ttm_bo_validate(&bo->tbo, &placement, &ctx);
>> +	if (r) {
>> +		DRM_ERROR("Failed to invalidate DMA-buf import (%d))\n", r);
>> +		return;
>> +	}
>> +
>> +	for (bo_base = bo->vm_bo; bo_base; bo_base = bo_base->next) {
>> +		struct amdgpu_vm *vm = bo_base->vm;
>> +		struct dma_resv *resv = vm->root.base.bo->tbo.base.resv;
>> +
>> +		if (ticket) {
> Yeah so this is kinda why I've been a total pain about the exact semantics
> of the move_notify hook. I think we should flat-out require that importers
> _always_ have a ticket attach when they call this, and that they can cope
> with additional locks being taken (i.e. full EDEADLCK) handling.

That is pretty much exactly my thinking as well.

And is also the sole reason why I started looking into the ww_mutex 
cursor handling a while back (e.g. the initial version with the horrible 
macro hack).

But this is really really hard to get right. So my thinking for now is 
to push this series upstream to at least unblock my ongoing P2P work.

> Simplest way to force that contract is to add a dummy 2nd ww_mutex lock to
> the dma_resv object, which we then can take #ifdef
> CONFIG_WW_MUTEX_SLOWPATH_DEBUG. Plus mabye a WARN_ON(!ticket).
>
> Now the real disaster is how we handle deadlocks. Two issues:
>
> - Ideally we'd keep any lock we've taken locked until the end, it helps
>    needless backoffs. I've played around a bit with that but not even poc
>    level, just an idea:
>
> https://cgit.freedesktop.org/~danvet/drm/commit/?id=b1799c5a0f02df9e1bb08d27be37331255ab7582
>
>    Idea is essentially to track a list of objects we had to lock as part of
>    the ttm_bo_validate of the main object.
>
> - Second one is if we get a EDEADLCK on one of these sublocks (like the
>    one here). We need to pass that up the entire callchain, including a
>    temporary reference (we have to drop locks to do the ww_mutex_lock_slow
>    call), and need a custom callback to drop that temporary reference
>    (since that's all driver specific, might even be internal ww_mutex and
>    not anything remotely looking like a normal dma_buf). This probably
>    needs the exec util helpers from ttm, but at the dma_resv level, so that
>    we can do something like this:
>
> struct dma_resv_ticket {
> 	struct ww_acquire_ctx base;
>
> 	/* can be set by anyone (including other drivers) that got hold of
> 	 * this ticket and had to acquire some new lock. This lock might
> 	 * protect anything, including driver-internal stuff, and isn't
> 	 * required to be a dma_buf or even just a dma_resv. */
> 	struct ww_mutex *contended_lock;
>
> 	/* callback which the driver (which might be a dma-buf exporter
> 	 * and not matching the driver that started this locking ticket)
> 	 * sets together with @contended_lock, for the main driver to drop
> 	 * when it calls dma_resv_unlock on the contended_lock. */
> 	void (drop_ref*)(struct ww_mutex *contended_lock);
> };

My initial thinking was to make all of this part of the core ww_mutex 
implementation, but then I quickly found that this won't work.

> This is all supremely nasty (also ttm_bo_validate would need to be
> improved to handle these sublocks and random new objects that could force
> a ww_mutex_lock_slow).

The next idea was to have it based on dma_resv objects, but as you also 
figured out you then need to drop the reference to the contended lock 
somehow...

So my current working plan was to use GEM object to avoid the callback...

> Plan B would be to throw our hands into and declare that "move_notify is
> best effort only and can fail for any reason". Exactly like ttm eviction
> currently does, even with all your hacks to do at least some dma_resv_lock
> (but not the full slowpath).

I would seriously NAK that. We have tried that with TTM and the whole 
idea is just braindead.

You can use trylock based eviction for things like best effort shrinker 
callbacks. But that's pretty much it.

> Given how much "fun" you have with all the low memory handling and ttm
> fundamentally being best-effort only (despite that dma_resv would allow us
> to do this right, with some work) I'm not sure that's a good idea to
> extend to a cross-driver interface. Personally I'd lean towards fixing
> this first fully (in ttm/amdgpu), and then using that to implement
> move_notify correctly.

Well have you seen this: https://fosdem.org/2020/schedule/event/ttm/ :)

> Or just add an int return value here and mandate that importers must
> handle eviction failures. Exactly like ttm_mem_evict_first can currently
> still fail for various reasons.
>
> (Sorry this isn't exactly the mail you hoped for)

Well it's actually the mail I expected. I'm thinking about exactly those 
problems for over a year now as well.

For the rather specific amdgpu case I could work around that by 
utilizing the HMM work to invalidate page tables on the fly, but that 
doesn't really help with memory management in general.

So YES, I totally agree that we need some sort of GEM execution context 
or something like this to lock buffers on the fly as we try to make room 
for others.

Regards,
Christian.

>
> Cheers, Daniel
>
>> +			/* When we get an error here it means that somebody
>> +			 * else is holding the VM lock and updating page tables
>> +			 * So we can just continue here.
>> +			 */
>> +			r = dma_resv_lock(resv, ticket);
>> +			if (r)
>> +				continue;
>> +
>> +		} else {
>> +			/* TODO: This is more problematic and we actually need
>> +			 * to allow page tables updates without holding the
>> +			 * lock.
>> +			 */
>> +			if (!dma_resv_trylock(resv))
>> +				continue;
>> +		}
>> +
>> +		r = amdgpu_vm_clear_freed(adev, vm, NULL);
>> +		if (!r)
>> +			r = amdgpu_vm_handle_moved(adev, vm);
>> +
>> +		if (r && r != -EBUSY)
>> +			DRM_ERROR("Failed to invalidate VM page tables (%d))\n",
>> +				  r);
>> +
>> +		dma_resv_unlock(resv);
>> +	}
>> +}
>> +
>>   static const struct dma_buf_attach_ops amdgpu_dma_buf_attach_ops = {
>> +	.move_notify = amdgpu_dma_buf_move_notify
>>   };
>>   
>>   /**
>> @@ -489,7 +553,7 @@ struct drm_gem_object *amdgpu_gem_prime_import(struct drm_device *dev,
>>   		return obj;
>>   
>>   	attach = dma_buf_dynamic_attach(dma_buf, dev->dev,
>> -					&amdgpu_dma_buf_attach_ops, NULL);
>> +					&amdgpu_dma_buf_attach_ops, obj);
>>   	if (IS_ERR(attach)) {
>>   		drm_gem_object_put(obj);
>>   		return ERR_CAST(attach);
>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
>> index 8ae260822908..8c480c898b0d 100644
>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
>> @@ -926,6 +926,9 @@ int amdgpu_bo_pin_restricted(struct amdgpu_bo *bo, u32 domain,
>>   		return 0;
>>   	}
>>   
>> +	if (bo->tbo.base.import_attach)
>> +		dma_buf_pin(bo->tbo.base.import_attach);
>> +
>>   	bo->flags |= AMDGPU_GEM_CREATE_VRAM_CONTIGUOUS;
>>   	/* force to pin into visible video ram */
>>   	if (!(bo->flags & AMDGPU_GEM_CREATE_NO_CPU_ACCESS))
>> @@ -1009,6 +1012,9 @@ int amdgpu_bo_unpin(struct amdgpu_bo *bo)
>>   
>>   	amdgpu_bo_subtract_pin_size(bo);
>>   
>> +	if (bo->tbo.base.import_attach)
>> +		dma_buf_unpin(bo->tbo.base.import_attach);
>> +
>>   	for (i = 0; i < bo->placement.num_placement; i++) {
>>   		bo->placements[i].lpfn = 0;
>>   		bo->placements[i].flags &= ~TTM_PL_FLAG_NO_EVICT;
>> -- 
>> 2.17.1
>>
Daniel Vetter Feb. 17, 2020, 7:38 p.m. UTC | #3
On Mon, Feb 17, 2020 at 7:58 PM Christian König
<ckoenig.leichtzumerken@gmail.com> wrote:
>
> Am 17.02.20 um 18:55 schrieb Daniel Vetter:
> > On Mon, Feb 17, 2020 at 04:45:09PM +0100, Christian König wrote:
> >> Implement the importer side of unpinned DMA-buf handling.
> >>
> >> v2: update page tables immediately
> >>
> >> Signed-off-by: Christian König <christian.koenig@amd.com>
> >> ---
> >>   drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 66 ++++++++++++++++++++-
> >>   drivers/gpu/drm/amd/amdgpu/amdgpu_object.c  |  6 ++
> >>   2 files changed, 71 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> >> index 770baba621b3..48de7624d49c 100644
> >> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> >> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> >> @@ -453,7 +453,71 @@ amdgpu_dma_buf_create_obj(struct drm_device *dev, struct dma_buf *dma_buf)
> >>      return ERR_PTR(ret);
> >>   }
> >>
> >> +/**
> >> + * amdgpu_dma_buf_move_notify - &attach.move_notify implementation
> >> + *
> >> + * @attach: the DMA-buf attachment
> >> + *
> >> + * Invalidate the DMA-buf attachment, making sure that the we re-create the
> >> + * mapping before the next use.
> >> + */
> >> +static void
> >> +amdgpu_dma_buf_move_notify(struct dma_buf_attachment *attach)
> >> +{
> >> +    struct drm_gem_object *obj = attach->importer_priv;
> >> +    struct ww_acquire_ctx *ticket = dma_resv_locking_ctx(obj->resv);
> >> +    struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
> >> +    struct amdgpu_device *adev = amdgpu_ttm_adev(bo->tbo.bdev);
> >> +    struct ttm_operation_ctx ctx = { false, false };
> >> +    struct ttm_placement placement = {};
> >> +    struct amdgpu_vm_bo_base *bo_base;
> >> +    int r;
> >> +
> >> +    if (bo->tbo.mem.mem_type == TTM_PL_SYSTEM)
> >> +            return;
> >> +
> >> +    r = ttm_bo_validate(&bo->tbo, &placement, &ctx);
> >> +    if (r) {
> >> +            DRM_ERROR("Failed to invalidate DMA-buf import (%d))\n", r);
> >> +            return;
> >> +    }
> >> +
> >> +    for (bo_base = bo->vm_bo; bo_base; bo_base = bo_base->next) {
> >> +            struct amdgpu_vm *vm = bo_base->vm;
> >> +            struct dma_resv *resv = vm->root.base.bo->tbo.base.resv;
> >> +
> >> +            if (ticket) {
> > Yeah so this is kinda why I've been a total pain about the exact semantics
> > of the move_notify hook. I think we should flat-out require that importers
> > _always_ have a ticket attach when they call this, and that they can cope
> > with additional locks being taken (i.e. full EDEADLCK) handling.
>
> That is pretty much exactly my thinking as well.
>
> And is also the sole reason why I started looking into the ww_mutex
> cursor handling a while back (e.g. the initial version with the horrible
> macro hack).
>
> But this is really really hard to get right. So my thinking for now is
> to push this series upstream to at least unblock my ongoing P2P work.

Hm, but at least the move_notify stuff and the locking nightmare
around that feels rushed if we just push that. Otoh it's indeed
getting painful, and we'll probably have another few rounds of
headaches to sort this all out. What about a

config EXPERIMENTAL_DYNAMIC_DMA_BUF
    depends on BROKEN

Wrapped around the new ->move_notify hook, plus all relevant code?
That way you can land at least something, in-tree refactoring might
become easier with at least some example of what it needs to achieve.
But we're also not tricking anyone into believing that this is
production ready.

> > Simplest way to force that contract is to add a dummy 2nd ww_mutex lock to
> > the dma_resv object, which we then can take #ifdef
> > CONFIG_WW_MUTEX_SLOWPATH_DEBUG. Plus mabye a WARN_ON(!ticket).
> >
> > Now the real disaster is how we handle deadlocks. Two issues:
> >
> > - Ideally we'd keep any lock we've taken locked until the end, it helps
> >    needless backoffs. I've played around a bit with that but not even poc
> >    level, just an idea:
> >
> > https://cgit.freedesktop.org/~danvet/drm/commit/?id=b1799c5a0f02df9e1bb08d27be37331255ab7582
> >
> >    Idea is essentially to track a list of objects we had to lock as part of
> >    the ttm_bo_validate of the main object.
> >
> > - Second one is if we get a EDEADLCK on one of these sublocks (like the
> >    one here). We need to pass that up the entire callchain, including a
> >    temporary reference (we have to drop locks to do the ww_mutex_lock_slow
> >    call), and need a custom callback to drop that temporary reference
> >    (since that's all driver specific, might even be internal ww_mutex and
> >    not anything remotely looking like a normal dma_buf). This probably
> >    needs the exec util helpers from ttm, but at the dma_resv level, so that
> >    we can do something like this:
> >
> > struct dma_resv_ticket {
> >       struct ww_acquire_ctx base;
> >
> >       /* can be set by anyone (including other drivers) that got hold of
> >        * this ticket and had to acquire some new lock. This lock might
> >        * protect anything, including driver-internal stuff, and isn't
> >        * required to be a dma_buf or even just a dma_resv. */
> >       struct ww_mutex *contended_lock;
> >
> >       /* callback which the driver (which might be a dma-buf exporter
> >        * and not matching the driver that started this locking ticket)
> >        * sets together with @contended_lock, for the main driver to drop
> >        * when it calls dma_resv_unlock on the contended_lock. */
> >       void (drop_ref*)(struct ww_mutex *contended_lock);
> > };
>
> My initial thinking was to make all of this part of the core ww_mutex
> implementation, but then I quickly found that this won't work.
>
> > This is all supremely nasty (also ttm_bo_validate would need to be
> > improved to handle these sublocks and random new objects that could force
> > a ww_mutex_lock_slow).
>
> The next idea was to have it based on dma_resv objects, but as you also
> figured out you then need to drop the reference to the contended lock
> somehow...
>
> So my current working plan was to use GEM object to avoid the callback...

I've heard noise that someone is looking into adding dynamic dma-buf
support to stuff like rdma drivers. Because interconnects and big
machines. Plus feels a bit awkward to mandate a gem library if you
want to use dynamic dma-buf support in your driver. Hence why I think
something around dma_resv (but with enough flexibility that it doesn't
insist that the contending lock must be a dma_resv itself).

> > Plan B would be to throw our hands into and declare that "move_notify is
> > best effort only and can fail for any reason". Exactly like ttm eviction
> > currently does, even with all your hacks to do at least some dma_resv_lock
> > (but not the full slowpath).
>
> I would seriously NAK that. We have tried that with TTM and the whole
> idea is just braindead.
>
> You can use trylock based eviction for things like best effort shrinker
> callbacks. But that's pretty much it.

Yeah trylock works pretty well for balancing caches, right up to the
point where you actually have to shrink stuff. Then suddenly all the
locks are contended because everyone is running low on memory :-/ We
have glorious amounts of experience with our best effort system memory
shrinker in i915 ...

> > Given how much "fun" you have with all the low memory handling and ttm
> > fundamentally being best-effort only (despite that dma_resv would allow us
> > to do this right, with some work) I'm not sure that's a good idea to
> > extend to a cross-driver interface. Personally I'd lean towards fixing
> > this first fully (in ttm/amdgpu), and then using that to implement
> > move_notify correctly.
>
> Well have you seen this: https://fosdem.org/2020/schedule/event/ttm/ :)

Oh cool, I was waiting for the upload. Will watch asap. btw slides somewhere?

> > Or just add an int return value here and mandate that importers must
> > handle eviction failures. Exactly like ttm_mem_evict_first can currently
> > still fail for various reasons.
> >
> > (Sorry this isn't exactly the mail you hoped for)
>
> Well it's actually the mail I expected. I'm thinking about exactly those
> problems for over a year now as well.
>
> For the rather specific amdgpu case I could work around that by
> utilizing the HMM work to invalidate page tables on the fly, but that
> doesn't really help with memory management in general.

Yeah, so move_notify is maybe solveable with better hw and hmm, but
there's other scenarios where I think the cross-driver ww_mutex
locking will be needed, for fundamental reasons. Scenario:
- a bunch of gpus in pcie slots, all in the same machine
- because pcie is slot a nice interconnect (iirc you guys call yours
xgmi or something like that)
- working sets that are bigger than vram of a single gpu
- lots of buffer sharing ofc

1. So driver has an imported dma-buf, currently not mapped anywhere
because ti got thrown out (or first use).
2. Importer calls dma_buf_map_attachement
3. Exporter realizes there's a nice xgmi link and p2p would be much
better if that object is in vram.
4. Exporter does ttm_bo_validate or equivalent to get the bo into
vram, including eviction and lots of locking
5. In turn this might bite back to the importer through some
move_notify of objects still mapped, but at the end of the lru.

So ->move_notify might not be the worst, eventually I think we'll need
the full locking dance across drivers (or at least across drm_device
instances, there might be internal upcasting going on so you get your
buffers placed in the right vram and all that directly).

> So YES, I totally agree that we need some sort of GEM execution context
> or something like this to lock buffers on the fly as we try to make room
> for others.

So what's the plan? Merge current series (with the bikesheds address)
under this CONFIG_EXPERIMENTAL_DYN_DMABUF and then see where we land
from there? Trying to get all the pieces lined up out of tree feels
like it's going to be too much :-/
-Daniel

>
> Regards,
> Christian.
>
> >
> > Cheers, Daniel
> >
> >> +                    /* When we get an error here it means that somebody
> >> +                     * else is holding the VM lock and updating page tables
> >> +                     * So we can just continue here.
> >> +                     */
> >> +                    r = dma_resv_lock(resv, ticket);
> >> +                    if (r)
> >> +                            continue;
> >> +
> >> +            } else {
> >> +                    /* TODO: This is more problematic and we actually need
> >> +                     * to allow page tables updates without holding the
> >> +                     * lock.
> >> +                     */
> >> +                    if (!dma_resv_trylock(resv))
> >> +                            continue;
> >> +            }
> >> +
> >> +            r = amdgpu_vm_clear_freed(adev, vm, NULL);
> >> +            if (!r)
> >> +                    r = amdgpu_vm_handle_moved(adev, vm);
> >> +
> >> +            if (r && r != -EBUSY)
> >> +                    DRM_ERROR("Failed to invalidate VM page tables (%d))\n",
> >> +                              r);
> >> +
> >> +            dma_resv_unlock(resv);
> >> +    }
> >> +}
> >> +
> >>   static const struct dma_buf_attach_ops amdgpu_dma_buf_attach_ops = {
> >> +    .move_notify = amdgpu_dma_buf_move_notify
> >>   };
> >>
> >>   /**
> >> @@ -489,7 +553,7 @@ struct drm_gem_object *amdgpu_gem_prime_import(struct drm_device *dev,
> >>              return obj;
> >>
> >>      attach = dma_buf_dynamic_attach(dma_buf, dev->dev,
> >> -                                    &amdgpu_dma_buf_attach_ops, NULL);
> >> +                                    &amdgpu_dma_buf_attach_ops, obj);
> >>      if (IS_ERR(attach)) {
> >>              drm_gem_object_put(obj);
> >>              return ERR_CAST(attach);
> >> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
> >> index 8ae260822908..8c480c898b0d 100644
> >> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
> >> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
> >> @@ -926,6 +926,9 @@ int amdgpu_bo_pin_restricted(struct amdgpu_bo *bo, u32 domain,
> >>              return 0;
> >>      }
> >>
> >> +    if (bo->tbo.base.import_attach)
> >> +            dma_buf_pin(bo->tbo.base.import_attach);
> >> +
> >>      bo->flags |= AMDGPU_GEM_CREATE_VRAM_CONTIGUOUS;
> >>      /* force to pin into visible video ram */
> >>      if (!(bo->flags & AMDGPU_GEM_CREATE_NO_CPU_ACCESS))
> >> @@ -1009,6 +1012,9 @@ int amdgpu_bo_unpin(struct amdgpu_bo *bo)
> >>
> >>      amdgpu_bo_subtract_pin_size(bo);
> >>
> >> +    if (bo->tbo.base.import_attach)
> >> +            dma_buf_unpin(bo->tbo.base.import_attach);
> >> +
> >>      for (i = 0; i < bo->placement.num_placement; i++) {
> >>              bo->placements[i].lpfn = 0;
> >>              bo->placements[i].flags &= ~TTM_PL_FLAG_NO_EVICT;
> >> --
> >> 2.17.1
> >>
>
Christian König Feb. 18, 2020, 10:42 a.m. UTC | #4
Am 17.02.20 um 20:38 schrieb Daniel Vetter:
> On Mon, Feb 17, 2020 at 7:58 PM Christian König
> <ckoenig.leichtzumerken@gmail.com> wrote:
>> Am 17.02.20 um 18:55 schrieb Daniel Vetter:
>>> On Mon, Feb 17, 2020 at 04:45:09PM +0100, Christian König wrote:
>>> [SNIP]
>> And is also the sole reason why I started looking into the ww_mutex
>> cursor handling a while back (e.g. the initial version with the horrible
>> macro hack).
>>
>> But this is really really hard to get right. So my thinking for now is
>> to push this series upstream to at least unblock my ongoing P2P work.
> Hm, but at least the move_notify stuff and the locking nightmare
> around that feels rushed if we just push that. Otoh it's indeed
> getting painful, and we'll probably have another few rounds of
> headaches to sort this all out. What about a
>
> config EXPERIMENTAL_DYNAMIC_DMA_BUF
>      depends on BROKEN
>
> Wrapped around the new ->move_notify hook, plus all relevant code?

Oh, that is a really good idea and trivial to to do.

>> My initial thinking was to make all of this part of the core ww_mutex
>> implementation, but then I quickly found that this won't work.
>>
>>> This is all supremely nasty (also ttm_bo_validate would need to be
>>> improved to handle these sublocks and random new objects that could force
>>> a ww_mutex_lock_slow).
>> The next idea was to have it based on dma_resv objects, but as you also
>> figured out you then need to drop the reference to the contended lock
>> somehow...
>>
>> So my current working plan was to use GEM object to avoid the callback...
> I've heard noise that someone is looking into adding dynamic dma-buf
> support to stuff like rdma drivers. Because interconnects and big
> machines. Plus feels a bit awkward to mandate a gem library if you
> want to use dynamic dma-buf support in your driver. Hence why I think
> something around dma_resv (but with enough flexibility that it doesn't
> insist that the contending lock must be a dma_resv itself).

Ok, good to know. So to hell with the idea of using a GEM object.

But this also means that we can't do this with a single drop_ref() 
callback in the context because the context might contains different 
objects of all kind.....

> [SNIP]
>
> Oh cool, I was waiting for the upload. Will watch asap. btw slides somewhere?

Attached.

>> [SNIP]
>> For the rather specific amdgpu case I could work around that by
>> utilizing the HMM work to invalidate page tables on the fly, but that
>> doesn't really help with memory management in general.
> Yeah, so move_notify is maybe solveable with better hw and hmm,

You don't even need better hardware and HMM.

All you need to do is clever locking because in this case you will never 
export page tables to other devices.

>   but
> there's other scenarios where I think the cross-driver ww_mutex
> locking will be needed, for fundamental reasons. Scenario:
> - a bunch of gpus in pcie slots, all in the same machine
> - because pcie is slot a nice interconnect (iirc you guys call yours
> xgmi or something like that)
> - working sets that are bigger than vram of a single gpu
> - lots of buffer sharing ofc

Yeah, completely agree. The issue with the page tables is actually a 
rather specific use case.

> 1. So driver has an imported dma-buf, currently not mapped anywhere
> because ti got thrown out (or first use).
> 2. Importer calls dma_buf_map_attachement
> 3. Exporter realizes there's a nice xgmi link and p2p would be much
> better if that object is in vram.
> 4. Exporter does ttm_bo_validate or equivalent to get the bo into
> vram, including eviction and lots of locking
> 5. In turn this might bite back to the importer through some
> move_notify of objects still mapped, but at the end of the lru.
>
> So ->move_notify might not be the worst, eventually I think we'll need
> the full locking dance across drivers (or at least across drm_device
> instances, there might be internal upcasting going on so you get your
> buffers placed in the right vram and all that directly).

YES, EXACTLY! That's the reason why I'm working on this stuff and not 
try to get P2P/XGMI/etc.. upstream directly without it.

I mean using P2P without all this is certainly possible, but sooner or 
later your memory management will just fall apart.

>> So YES, I totally agree that we need some sort of GEM execution context
>> or something like this to lock buffers on the fly as we try to make room
>> for others.
> So what's the plan? Merge current series (with the bikesheds address)
> under this CONFIG_EXPERIMENTAL_DYN_DMABUF and then see where we land
> from there? Trying to get all the pieces lined up out of tree feels
> like it's going to be too much :-/

At least I hoped for something like that.

Developing this out of tree and especially since I have this only as a 
background task turned out to be delaying things over and over again.

This way I can get it upstream (even when still under experimental flag) 
and start to convince our internal team/customers that we should work on 
investing time into this.

Thanks,
Christian.

> -Daniel
>
>> Regards,
>> Christian.
>>
>>> Cheers, Daniel
>>>
>>>> +                    /* When we get an error here it means that somebody
>>>> +                     * else is holding the VM lock and updating page tables
>>>> +                     * So we can just continue here.
>>>> +                     */
>>>> +                    r = dma_resv_lock(resv, ticket);
>>>> +                    if (r)
>>>> +                            continue;
>>>> +
>>>> +            } else {
>>>> +                    /* TODO: This is more problematic and we actually need
>>>> +                     * to allow page tables updates without holding the
>>>> +                     * lock.
>>>> +                     */
>>>> +                    if (!dma_resv_trylock(resv))
>>>> +                            continue;
>>>> +            }
>>>> +
>>>> +            r = amdgpu_vm_clear_freed(adev, vm, NULL);
>>>> +            if (!r)
>>>> +                    r = amdgpu_vm_handle_moved(adev, vm);
>>>> +
>>>> +            if (r && r != -EBUSY)
>>>> +                    DRM_ERROR("Failed to invalidate VM page tables (%d))\n",
>>>> +                              r);
>>>> +
>>>> +            dma_resv_unlock(resv);
>>>> +    }
>>>> +}
>>>> +
>>>>    static const struct dma_buf_attach_ops amdgpu_dma_buf_attach_ops = {
>>>> +    .move_notify = amdgpu_dma_buf_move_notify
>>>>    };
>>>>
>>>>    /**
>>>> @@ -489,7 +553,7 @@ struct drm_gem_object *amdgpu_gem_prime_import(struct drm_device *dev,
>>>>               return obj;
>>>>
>>>>       attach = dma_buf_dynamic_attach(dma_buf, dev->dev,
>>>> -                                    &amdgpu_dma_buf_attach_ops, NULL);
>>>> +                                    &amdgpu_dma_buf_attach_ops, obj);
>>>>       if (IS_ERR(attach)) {
>>>>               drm_gem_object_put(obj);
>>>>               return ERR_CAST(attach);
>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
>>>> index 8ae260822908..8c480c898b0d 100644
>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
>>>> @@ -926,6 +926,9 @@ int amdgpu_bo_pin_restricted(struct amdgpu_bo *bo, u32 domain,
>>>>               return 0;
>>>>       }
>>>>
>>>> +    if (bo->tbo.base.import_attach)
>>>> +            dma_buf_pin(bo->tbo.base.import_attach);
>>>> +
>>>>       bo->flags |= AMDGPU_GEM_CREATE_VRAM_CONTIGUOUS;
>>>>       /* force to pin into visible video ram */
>>>>       if (!(bo->flags & AMDGPU_GEM_CREATE_NO_CPU_ACCESS))
>>>> @@ -1009,6 +1012,9 @@ int amdgpu_bo_unpin(struct amdgpu_bo *bo)
>>>>
>>>>       amdgpu_bo_subtract_pin_size(bo);
>>>>
>>>> +    if (bo->tbo.base.import_attach)
>>>> +            dma_buf_unpin(bo->tbo.base.import_attach);
>>>> +
>>>>       for (i = 0; i < bo->placement.num_placement; i++) {
>>>>               bo->placements[i].lpfn = 0;
>>>>               bo->placements[i].flags &= ~TTM_PL_FLAG_NO_EVICT;
>>>> --
>>>> 2.17.1
>>>>
>
Thomas Hellström (VMware) Feb. 18, 2020, 8:17 p.m. UTC | #5
On 2/17/20 6:55 PM, Daniel Vetter wrote:
> On Mon, Feb 17, 2020 at 04:45:09PM +0100, Christian König wrote:
>> Implement the importer side of unpinned DMA-buf handling.
>>
>> v2: update page tables immediately
>>
>> Signed-off-by: Christian König <christian.koenig@amd.com>
>> ---
>>   drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 66 ++++++++++++++++++++-
>>   drivers/gpu/drm/amd/amdgpu/amdgpu_object.c  |  6 ++
>>   2 files changed, 71 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>> index 770baba621b3..48de7624d49c 100644
>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>> @@ -453,7 +453,71 @@ amdgpu_dma_buf_create_obj(struct drm_device *dev, struct dma_buf *dma_buf)
>>   	return ERR_PTR(ret);
>>   }
>>   
>> +/**
>> + * amdgpu_dma_buf_move_notify - &attach.move_notify implementation
>> + *
>> + * @attach: the DMA-buf attachment
>> + *
>> + * Invalidate the DMA-buf attachment, making sure that the we re-create the
>> + * mapping before the next use.
>> + */
>> +static void
>> +amdgpu_dma_buf_move_notify(struct dma_buf_attachment *attach)
>> +{
>> +	struct drm_gem_object *obj = attach->importer_priv;
>> +	struct ww_acquire_ctx *ticket = dma_resv_locking_ctx(obj->resv);
>> +	struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
>> +	struct amdgpu_device *adev = amdgpu_ttm_adev(bo->tbo.bdev);
>> +	struct ttm_operation_ctx ctx = { false, false };
>> +	struct ttm_placement placement = {};
>> +	struct amdgpu_vm_bo_base *bo_base;
>> +	int r;
>> +
>> +	if (bo->tbo.mem.mem_type == TTM_PL_SYSTEM)
>> +		return;
>> +
>> +	r = ttm_bo_validate(&bo->tbo, &placement, &ctx);
>> +	if (r) {
>> +		DRM_ERROR("Failed to invalidate DMA-buf import (%d))\n", r);
>> +		return;
>> +	}
>> +
>> +	for (bo_base = bo->vm_bo; bo_base; bo_base = bo_base->next) {
>> +		struct amdgpu_vm *vm = bo_base->vm;
>> +		struct dma_resv *resv = vm->root.base.bo->tbo.base.resv;
>> +
>> +		if (ticket) {
> Yeah so this is kinda why I've been a total pain about the exact semantics
> of the move_notify hook. I think we should flat-out require that importers
> _always_ have a ticket attach when they call this, and that they can cope
> with additional locks being taken (i.e. full EDEADLCK) handling.
>
> Simplest way to force that contract is to add a dummy 2nd ww_mutex lock to
> the dma_resv object, which we then can take #ifdef
> CONFIG_WW_MUTEX_SLOWPATH_DEBUG. Plus mabye a WARN_ON(!ticket).
>
> Now the real disaster is how we handle deadlocks. Two issues:
>
> - Ideally we'd keep any lock we've taken locked until the end, it helps
>    needless backoffs. I've played around a bit with that but not even poc
>    level, just an idea:
>
> https://cgit.freedesktop.org/~danvet/drm/commit/?id=b1799c5a0f02df9e1bb08d27be37331255ab7582
>
>    Idea is essentially to track a list of objects we had to lock as part of
>    the ttm_bo_validate of the main object.
>
> - Second one is if we get a EDEADLCK on one of these sublocks (like the
>    one here). We need to pass that up the entire callchain, including a
>    temporary reference (we have to drop locks to do the ww_mutex_lock_slow
>    call), and need a custom callback to drop that temporary reference
>    (since that's all driver specific, might even be internal ww_mutex and
>    not anything remotely looking like a normal dma_buf). This probably
>    needs the exec util helpers from ttm, but at the dma_resv level, so that
>    we can do something like this:
>
> struct dma_resv_ticket {
> 	struct ww_acquire_ctx base;
>
> 	/* can be set by anyone (including other drivers) that got hold of
> 	 * this ticket and had to acquire some new lock. This lock might
> 	 * protect anything, including driver-internal stuff, and isn't
> 	 * required to be a dma_buf or even just a dma_resv. */
> 	struct ww_mutex *contended_lock;
>
> 	/* callback which the driver (which might be a dma-buf exporter
> 	 * and not matching the driver that started this locking ticket)
> 	 * sets together with @contended_lock, for the main driver to drop
> 	 * when it calls dma_resv_unlock on the contended_lock. */
> 	void (drop_ref*)(struct ww_mutex *contended_lock);
> };
>
> This is all supremely nasty (also ttm_bo_validate would need to be
> improved to handle these sublocks and random new objects that could force
> a ww_mutex_lock_slow).
>
Just a short comment on this:

Neither the currently used wait-die or the wound-wait algorithm 
*strictly* requires a slow lock on the contended lock. For wait-die it's 
just very convenient since it makes us sleep instead of spinning with 
-EDEADLK on the contended lock. For wound-wait IIRC one could just 
immediately restart the whole locking transaction after an -EDEADLK, and 
the transaction would automatically end up waiting on the contended 
lock, provided the mutex lock stealing is not allowed. There is however 
a possibility that the transaction will be wounded again on another 
lock, taken before the contended lock, but I think there are ways to 
improve the wound-wait algorithm to reduce that probability.

So in short, choosing the wound-wait algorithm instead of wait-die and 
perhaps modifying the ww mutex code somewhat would probably help passing 
an -EDEADLK up the call chain without requiring passing the contended 
lock, as long as each locker releases its own locks when receiving an 
-EDEADLK.

/Thomas
Daniel Vetter Feb. 18, 2020, 9:01 p.m. UTC | #6
On Tue, Feb 18, 2020 at 9:17 PM Thomas Hellström (VMware)
<thomas_os@shipmail.org> wrote:
>
> On 2/17/20 6:55 PM, Daniel Vetter wrote:
> > On Mon, Feb 17, 2020 at 04:45:09PM +0100, Christian König wrote:
> >> Implement the importer side of unpinned DMA-buf handling.
> >>
> >> v2: update page tables immediately
> >>
> >> Signed-off-by: Christian König <christian.koenig@amd.com>
> >> ---
> >>   drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 66 ++++++++++++++++++++-
> >>   drivers/gpu/drm/amd/amdgpu/amdgpu_object.c  |  6 ++
> >>   2 files changed, 71 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> >> index 770baba621b3..48de7624d49c 100644
> >> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> >> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> >> @@ -453,7 +453,71 @@ amdgpu_dma_buf_create_obj(struct drm_device *dev, struct dma_buf *dma_buf)
> >>      return ERR_PTR(ret);
> >>   }
> >>
> >> +/**
> >> + * amdgpu_dma_buf_move_notify - &attach.move_notify implementation
> >> + *
> >> + * @attach: the DMA-buf attachment
> >> + *
> >> + * Invalidate the DMA-buf attachment, making sure that the we re-create the
> >> + * mapping before the next use.
> >> + */
> >> +static void
> >> +amdgpu_dma_buf_move_notify(struct dma_buf_attachment *attach)
> >> +{
> >> +    struct drm_gem_object *obj = attach->importer_priv;
> >> +    struct ww_acquire_ctx *ticket = dma_resv_locking_ctx(obj->resv);
> >> +    struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
> >> +    struct amdgpu_device *adev = amdgpu_ttm_adev(bo->tbo.bdev);
> >> +    struct ttm_operation_ctx ctx = { false, false };
> >> +    struct ttm_placement placement = {};
> >> +    struct amdgpu_vm_bo_base *bo_base;
> >> +    int r;
> >> +
> >> +    if (bo->tbo.mem.mem_type == TTM_PL_SYSTEM)
> >> +            return;
> >> +
> >> +    r = ttm_bo_validate(&bo->tbo, &placement, &ctx);
> >> +    if (r) {
> >> +            DRM_ERROR("Failed to invalidate DMA-buf import (%d))\n", r);
> >> +            return;
> >> +    }
> >> +
> >> +    for (bo_base = bo->vm_bo; bo_base; bo_base = bo_base->next) {
> >> +            struct amdgpu_vm *vm = bo_base->vm;
> >> +            struct dma_resv *resv = vm->root.base.bo->tbo.base.resv;
> >> +
> >> +            if (ticket) {
> > Yeah so this is kinda why I've been a total pain about the exact semantics
> > of the move_notify hook. I think we should flat-out require that importers
> > _always_ have a ticket attach when they call this, and that they can cope
> > with additional locks being taken (i.e. full EDEADLCK) handling.
> >
> > Simplest way to force that contract is to add a dummy 2nd ww_mutex lock to
> > the dma_resv object, which we then can take #ifdef
> > CONFIG_WW_MUTEX_SLOWPATH_DEBUG. Plus mabye a WARN_ON(!ticket).
> >
> > Now the real disaster is how we handle deadlocks. Two issues:
> >
> > - Ideally we'd keep any lock we've taken locked until the end, it helps
> >    needless backoffs. I've played around a bit with that but not even poc
> >    level, just an idea:
> >
> > https://cgit.freedesktop.org/~danvet/drm/commit/?id=b1799c5a0f02df9e1bb08d27be37331255ab7582
> >
> >    Idea is essentially to track a list of objects we had to lock as part of
> >    the ttm_bo_validate of the main object.
> >
> > - Second one is if we get a EDEADLCK on one of these sublocks (like the
> >    one here). We need to pass that up the entire callchain, including a
> >    temporary reference (we have to drop locks to do the ww_mutex_lock_slow
> >    call), and need a custom callback to drop that temporary reference
> >    (since that's all driver specific, might even be internal ww_mutex and
> >    not anything remotely looking like a normal dma_buf). This probably
> >    needs the exec util helpers from ttm, but at the dma_resv level, so that
> >    we can do something like this:
> >
> > struct dma_resv_ticket {
> >       struct ww_acquire_ctx base;
> >
> >       /* can be set by anyone (including other drivers) that got hold of
> >        * this ticket and had to acquire some new lock. This lock might
> >        * protect anything, including driver-internal stuff, and isn't
> >        * required to be a dma_buf or even just a dma_resv. */
> >       struct ww_mutex *contended_lock;
> >
> >       /* callback which the driver (which might be a dma-buf exporter
> >        * and not matching the driver that started this locking ticket)
> >        * sets together with @contended_lock, for the main driver to drop
> >        * when it calls dma_resv_unlock on the contended_lock. */
> >       void (drop_ref*)(struct ww_mutex *contended_lock);
> > };
> >
> > This is all supremely nasty (also ttm_bo_validate would need to be
> > improved to handle these sublocks and random new objects that could force
> > a ww_mutex_lock_slow).
> >
> Just a short comment on this:
>
> Neither the currently used wait-die or the wound-wait algorithm
> *strictly* requires a slow lock on the contended lock. For wait-die it's
> just very convenient since it makes us sleep instead of spinning with
> -EDEADLK on the contended lock. For wound-wait IIRC one could just
> immediately restart the whole locking transaction after an -EDEADLK, and
> the transaction would automatically end up waiting on the contended
> lock, provided the mutex lock stealing is not allowed. There is however
> a possibility that the transaction will be wounded again on another
> lock, taken before the contended lock, but I think there are ways to
> improve the wound-wait algorithm to reduce that probability.
>
> So in short, choosing the wound-wait algorithm instead of wait-die and
> perhaps modifying the ww mutex code somewhat would probably help passing
> an -EDEADLK up the call chain without requiring passing the contended
> lock, as long as each locker releases its own locks when receiving an
> -EDEADLK.

Hm this is kinda tempting, since rolling out the full backoff tricker
across driver boundaries is going to be real painful.

What I'm kinda worried about is the debug/validation checks we're
losing with this. The required backoff has this nice property that
ww_mutex debug code can check that we've fully unwound everything when
we should, that we've blocked on the right lock, and that we're
restarting everything without keeling over. Without that I think we
could end up with situations where a driver in the middle feels like
handling the EDEADLCK, which might go well most of the times (the
deadlock will probably be mostly within a given driver, not across).
Right up to the point where someone creates a deadlock across drivers,
and the lack of full rollback will be felt.

So not sure whether we can still keep all these debug/validation
checks, or whether this is a step too far towards clever tricks.

But definitely a neat idea ...
-Daniel
Thomas Hellström (VMware) Feb. 19, 2020, 6:42 a.m. UTC | #7
On 2/18/20 10:01 PM, Daniel Vetter wrote:
> On Tue, Feb 18, 2020 at 9:17 PM Thomas Hellström (VMware)
> <thomas_os@shipmail.org> wrote:
>> On 2/17/20 6:55 PM, Daniel Vetter wrote:
>>> On Mon, Feb 17, 2020 at 04:45:09PM +0100, Christian König wrote:
>>>> Implement the importer side of unpinned DMA-buf handling.
>>>>
>>>> v2: update page tables immediately
>>>>
>>>> Signed-off-by: Christian König <christian.koenig@amd.com>
>>>> ---
>>>>    drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 66 ++++++++++++++++++++-
>>>>    drivers/gpu/drm/amd/amdgpu/amdgpu_object.c  |  6 ++
>>>>    2 files changed, 71 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>> index 770baba621b3..48de7624d49c 100644
>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>> @@ -453,7 +453,71 @@ amdgpu_dma_buf_create_obj(struct drm_device *dev, struct dma_buf *dma_buf)
>>>>       return ERR_PTR(ret);
>>>>    }
>>>>
>>>> +/**
>>>> + * amdgpu_dma_buf_move_notify - &attach.move_notify implementation
>>>> + *
>>>> + * @attach: the DMA-buf attachment
>>>> + *
>>>> + * Invalidate the DMA-buf attachment, making sure that the we re-create the
>>>> + * mapping before the next use.
>>>> + */
>>>> +static void
>>>> +amdgpu_dma_buf_move_notify(struct dma_buf_attachment *attach)
>>>> +{
>>>> +    struct drm_gem_object *obj = attach->importer_priv;
>>>> +    struct ww_acquire_ctx *ticket = dma_resv_locking_ctx(obj->resv);
>>>> +    struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
>>>> +    struct amdgpu_device *adev = amdgpu_ttm_adev(bo->tbo.bdev);
>>>> +    struct ttm_operation_ctx ctx = { false, false };
>>>> +    struct ttm_placement placement = {};
>>>> +    struct amdgpu_vm_bo_base *bo_base;
>>>> +    int r;
>>>> +
>>>> +    if (bo->tbo.mem.mem_type == TTM_PL_SYSTEM)
>>>> +            return;
>>>> +
>>>> +    r = ttm_bo_validate(&bo->tbo, &placement, &ctx);
>>>> +    if (r) {
>>>> +            DRM_ERROR("Failed to invalidate DMA-buf import (%d))\n", r);
>>>> +            return;
>>>> +    }
>>>> +
>>>> +    for (bo_base = bo->vm_bo; bo_base; bo_base = bo_base->next) {
>>>> +            struct amdgpu_vm *vm = bo_base->vm;
>>>> +            struct dma_resv *resv = vm->root.base.bo->tbo.base.resv;
>>>> +
>>>> +            if (ticket) {
>>> Yeah so this is kinda why I've been a total pain about the exact semantics
>>> of the move_notify hook. I think we should flat-out require that importers
>>> _always_ have a ticket attach when they call this, and that they can cope
>>> with additional locks being taken (i.e. full EDEADLCK) handling.
>>>
>>> Simplest way to force that contract is to add a dummy 2nd ww_mutex lock to
>>> the dma_resv object, which we then can take #ifdef
>>> CONFIG_WW_MUTEX_SLOWPATH_DEBUG. Plus mabye a WARN_ON(!ticket).
>>>
>>> Now the real disaster is how we handle deadlocks. Two issues:
>>>
>>> - Ideally we'd keep any lock we've taken locked until the end, it helps
>>>     needless backoffs. I've played around a bit with that but not even poc
>>>     level, just an idea:
>>>
>>> https://cgit.freedesktop.org/~danvet/drm/commit/?id=b1799c5a0f02df9e1bb08d27be37331255ab7582
>>>
>>>     Idea is essentially to track a list of objects we had to lock as part of
>>>     the ttm_bo_validate of the main object.
>>>
>>> - Second one is if we get a EDEADLCK on one of these sublocks (like the
>>>     one here). We need to pass that up the entire callchain, including a
>>>     temporary reference (we have to drop locks to do the ww_mutex_lock_slow
>>>     call), and need a custom callback to drop that temporary reference
>>>     (since that's all driver specific, might even be internal ww_mutex and
>>>     not anything remotely looking like a normal dma_buf). This probably
>>>     needs the exec util helpers from ttm, but at the dma_resv level, so that
>>>     we can do something like this:
>>>
>>> struct dma_resv_ticket {
>>>        struct ww_acquire_ctx base;
>>>
>>>        /* can be set by anyone (including other drivers) that got hold of
>>>         * this ticket and had to acquire some new lock. This lock might
>>>         * protect anything, including driver-internal stuff, and isn't
>>>         * required to be a dma_buf or even just a dma_resv. */
>>>        struct ww_mutex *contended_lock;
>>>
>>>        /* callback which the driver (which might be a dma-buf exporter
>>>         * and not matching the driver that started this locking ticket)
>>>         * sets together with @contended_lock, for the main driver to drop
>>>         * when it calls dma_resv_unlock on the contended_lock. */
>>>        void (drop_ref*)(struct ww_mutex *contended_lock);
>>> };
>>>
>>> This is all supremely nasty (also ttm_bo_validate would need to be
>>> improved to handle these sublocks and random new objects that could force
>>> a ww_mutex_lock_slow).
>>>
>> Just a short comment on this:
>>
>> Neither the currently used wait-die or the wound-wait algorithm
>> *strictly* requires a slow lock on the contended lock. For wait-die it's
>> just very convenient since it makes us sleep instead of spinning with
>> -EDEADLK on the contended lock. For wound-wait IIRC one could just
>> immediately restart the whole locking transaction after an -EDEADLK, and
>> the transaction would automatically end up waiting on the contended
>> lock, provided the mutex lock stealing is not allowed. There is however
>> a possibility that the transaction will be wounded again on another
>> lock, taken before the contended lock, but I think there are ways to
>> improve the wound-wait algorithm to reduce that probability.
>>
>> So in short, choosing the wound-wait algorithm instead of wait-die and
>> perhaps modifying the ww mutex code somewhat would probably help passing
>> an -EDEADLK up the call chain without requiring passing the contended
>> lock, as long as each locker releases its own locks when receiving an
>> -EDEADLK.
> Hm this is kinda tempting, since rolling out the full backoff tricker
> across driver boundaries is going to be real painful.
>
> What I'm kinda worried about is the debug/validation checks we're
> losing with this. The required backoff has this nice property that
> ww_mutex debug code can check that we've fully unwound everything when
> we should, that we've blocked on the right lock, and that we're
> restarting everything without keeling over. Without that I think we
> could end up with situations where a driver in the middle feels like
> handling the EDEADLCK, which might go well most of the times (the
> deadlock will probably be mostly within a given driver, not across).
> Right up to the point where someone creates a deadlock across drivers,
> and the lack of full rollback will be felt.
>
> So not sure whether we can still keep all these debug/validation
> checks, or whether this is a step too far towards clever tricks.

I think we could definitely find a way to keep debugging to make sure 
everything is unwound before attempting to restart the locking 
transaction. But the debug check that we're restarting on the contended 
lock only really makes sense for wait-die, (and we could easily keep it 
for wait-die). The lock returning -EDEADLK for wound-wait may actually 
not be the contending lock but an arbitrary lock that the wounded 
transaction attempts to take after it is wounded.

So in the end IMO this is a tradeoff between added (possibly severe) 
locking complexity into dma-buf and not being able to switch back to 
wait-die efficiently if we need / want to do that.

/Thomas


>
> But definitely a neat idea ...
> -Daniel
Thomas Hellström (VMware) Feb. 20, 2020, 9:39 a.m. UTC | #8
On 2/19/20 7:42 AM, Thomas Hellström (VMware) wrote:
> On 2/18/20 10:01 PM, Daniel Vetter wrote:
>> On Tue, Feb 18, 2020 at 9:17 PM Thomas Hellström (VMware)
>> <thomas_os@shipmail.org> wrote:
>>> On 2/17/20 6:55 PM, Daniel Vetter wrote:
>>>> On Mon, Feb 17, 2020 at 04:45:09PM +0100, Christian König wrote:
>>>>> Implement the importer side of unpinned DMA-buf handling.
>>>>>
>>>>> v2: update page tables immediately
>>>>>
>>>>> Signed-off-by: Christian König <christian.koenig@amd.com>
>>>>> ---
>>>>>    drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 66 
>>>>> ++++++++++++++++++++-
>>>>>    drivers/gpu/drm/amd/amdgpu/amdgpu_object.c  |  6 ++
>>>>>    2 files changed, 71 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c 
>>>>> b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>>> index 770baba621b3..48de7624d49c 100644
>>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>>> @@ -453,7 +453,71 @@ amdgpu_dma_buf_create_obj(struct drm_device 
>>>>> *dev, struct dma_buf *dma_buf)
>>>>>       return ERR_PTR(ret);
>>>>>    }
>>>>>
>>>>> +/**
>>>>> + * amdgpu_dma_buf_move_notify - &attach.move_notify implementation
>>>>> + *
>>>>> + * @attach: the DMA-buf attachment
>>>>> + *
>>>>> + * Invalidate the DMA-buf attachment, making sure that the we 
>>>>> re-create the
>>>>> + * mapping before the next use.
>>>>> + */
>>>>> +static void
>>>>> +amdgpu_dma_buf_move_notify(struct dma_buf_attachment *attach)
>>>>> +{
>>>>> +    struct drm_gem_object *obj = attach->importer_priv;
>>>>> +    struct ww_acquire_ctx *ticket = dma_resv_locking_ctx(obj->resv);
>>>>> +    struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
>>>>> +    struct amdgpu_device *adev = amdgpu_ttm_adev(bo->tbo.bdev);
>>>>> +    struct ttm_operation_ctx ctx = { false, false };
>>>>> +    struct ttm_placement placement = {};
>>>>> +    struct amdgpu_vm_bo_base *bo_base;
>>>>> +    int r;
>>>>> +
>>>>> +    if (bo->tbo.mem.mem_type == TTM_PL_SYSTEM)
>>>>> +            return;
>>>>> +
>>>>> +    r = ttm_bo_validate(&bo->tbo, &placement, &ctx);
>>>>> +    if (r) {
>>>>> +            DRM_ERROR("Failed to invalidate DMA-buf import 
>>>>> (%d))\n", r);
>>>>> +            return;
>>>>> +    }
>>>>> +
>>>>> +    for (bo_base = bo->vm_bo; bo_base; bo_base = bo_base->next) {
>>>>> +            struct amdgpu_vm *vm = bo_base->vm;
>>>>> +            struct dma_resv *resv = vm->root.base.bo->tbo.base.resv;
>>>>> +
>>>>> +            if (ticket) {
>>>> Yeah so this is kinda why I've been a total pain about the exact 
>>>> semantics
>>>> of the move_notify hook. I think we should flat-out require that 
>>>> importers
>>>> _always_ have a ticket attach when they call this, and that they 
>>>> can cope
>>>> with additional locks being taken (i.e. full EDEADLCK) handling.
>>>>
>>>> Simplest way to force that contract is to add a dummy 2nd ww_mutex 
>>>> lock to
>>>> the dma_resv object, which we then can take #ifdef
>>>> CONFIG_WW_MUTEX_SLOWPATH_DEBUG. Plus mabye a WARN_ON(!ticket).
>>>>
>>>> Now the real disaster is how we handle deadlocks. Two issues:
>>>>
>>>> - Ideally we'd keep any lock we've taken locked until the end, it 
>>>> helps
>>>>     needless backoffs. I've played around a bit with that but not 
>>>> even poc
>>>>     level, just an idea:
>>>>
>>>> https://cgit.freedesktop.org/~danvet/drm/commit/?id=b1799c5a0f02df9e1bb08d27be37331255ab7582 
>>>>
>>>>
>>>>     Idea is essentially to track a list of objects we had to lock 
>>>> as part of
>>>>     the ttm_bo_validate of the main object.
>>>>
>>>> - Second one is if we get a EDEADLCK on one of these sublocks (like 
>>>> the
>>>>     one here). We need to pass that up the entire callchain, 
>>>> including a
>>>>     temporary reference (we have to drop locks to do the 
>>>> ww_mutex_lock_slow
>>>>     call), and need a custom callback to drop that temporary reference
>>>>     (since that's all driver specific, might even be internal 
>>>> ww_mutex and
>>>>     not anything remotely looking like a normal dma_buf). This 
>>>> probably
>>>>     needs the exec util helpers from ttm, but at the dma_resv 
>>>> level, so that
>>>>     we can do something like this:
>>>>
>>>> struct dma_resv_ticket {
>>>>        struct ww_acquire_ctx base;
>>>>
>>>>        /* can be set by anyone (including other drivers) that got 
>>>> hold of
>>>>         * this ticket and had to acquire some new lock. This lock 
>>>> might
>>>>         * protect anything, including driver-internal stuff, and isn't
>>>>         * required to be a dma_buf or even just a dma_resv. */
>>>>        struct ww_mutex *contended_lock;
>>>>
>>>>        /* callback which the driver (which might be a dma-buf exporter
>>>>         * and not matching the driver that started this locking 
>>>> ticket)
>>>>         * sets together with @contended_lock, for the main driver 
>>>> to drop
>>>>         * when it calls dma_resv_unlock on the contended_lock. */
>>>>        void (drop_ref*)(struct ww_mutex *contended_lock);
>>>> };
>>>>
>>>> This is all supremely nasty (also ttm_bo_validate would need to be
>>>> improved to handle these sublocks and random new objects that could 
>>>> force
>>>> a ww_mutex_lock_slow).
>>>>
>>> Just a short comment on this:
>>>
>>> Neither the currently used wait-die or the wound-wait algorithm
>>> *strictly* requires a slow lock on the contended lock. For wait-die 
>>> it's
>>> just very convenient since it makes us sleep instead of spinning with
>>> -EDEADLK on the contended lock. For wound-wait IIRC one could just
>>> immediately restart the whole locking transaction after an -EDEADLK, 
>>> and
>>> the transaction would automatically end up waiting on the contended
>>> lock, provided the mutex lock stealing is not allowed. There is however
>>> a possibility that the transaction will be wounded again on another
>>> lock, taken before the contended lock, but I think there are ways to
>>> improve the wound-wait algorithm to reduce that probability.
>>>
>>> So in short, choosing the wound-wait algorithm instead of wait-die and
>>> perhaps modifying the ww mutex code somewhat would probably help 
>>> passing
>>> an -EDEADLK up the call chain without requiring passing the contended
>>> lock, as long as each locker releases its own locks when receiving an
>>> -EDEADLK.
>> Hm this is kinda tempting, since rolling out the full backoff tricker
>> across driver boundaries is going to be real painful.
>>
>> What I'm kinda worried about is the debug/validation checks we're
>> losing with this. The required backoff has this nice property that
>> ww_mutex debug code can check that we've fully unwound everything when
>> we should, that we've blocked on the right lock, and that we're
>> restarting everything without keeling over. Without that I think we
>> could end up with situations where a driver in the middle feels like
>> handling the EDEADLCK, which might go well most of the times (the
>> deadlock will probably be mostly within a given driver, not across).
>> Right up to the point where someone creates a deadlock across drivers,
>> and the lack of full rollback will be felt.
>>
>> So not sure whether we can still keep all these debug/validation
>> checks, or whether this is a step too far towards clever tricks.
>
> I think we could definitely find a way to keep debugging to make sure 
> everything is unwound before attempting to restart the locking 
> transaction. But the debug check that we're restarting on the 
> contended lock only really makes sense for wait-die, (and we could 
> easily keep it for wait-die). The lock returning -EDEADLK for 
> wound-wait may actually not be the contending lock but an arbitrary 
> lock that the wounded transaction attempts to take after it is wounded.
>
> So in the end IMO this is a tradeoff between added (possibly severe) 
> locking complexity into dma-buf and not being able to switch back to 
> wait-die efficiently if we need / want to do that.
>
> /Thomas

And as a consequence an interface *could* be:

*) We introduce functions

void ww_acquire_relax(struct ww_acquire_ctx *ctx);
int ww_acquire_relax_interruptible(struct ww_acquire_ctx *ctx);

that can be used instead of ww_mutex_lock_slow() in the absence of a 
contending lock to avoid spinning on -EDEADLK. While trying to take the 
contending lock is probably the best choice there are various second 
best approaches that can be explored, for example waiting on the 
contending acquire to finish or in the wound-wait case, perhaps do 
nothing. These functions will also help us keep the debugging.

*) A function returning -EDEADLK to a caller *must* have already 
released its own locks.

*) move_notify() explicitly takes a struct ww_acquire_ctx * to make sure 
there is no ambiguity. (I think it would be valuable if we could do the 
same for ttm_bo_validate()).

/Thomas
Daniel Vetter Feb. 20, 2020, 6:04 p.m. UTC | #9
On Thu, Feb 20, 2020 at 10:39:06AM +0100, Thomas Hellström (VMware) wrote:
> On 2/19/20 7:42 AM, Thomas Hellström (VMware) wrote:
> > On 2/18/20 10:01 PM, Daniel Vetter wrote:
> > > On Tue, Feb 18, 2020 at 9:17 PM Thomas Hellström (VMware)
> > > <thomas_os@shipmail.org> wrote:
> > > > On 2/17/20 6:55 PM, Daniel Vetter wrote:
> > > > > On Mon, Feb 17, 2020 at 04:45:09PM +0100, Christian König wrote:
> > > > > > Implement the importer side of unpinned DMA-buf handling.
> > > > > > 
> > > > > > v2: update page tables immediately
> > > > > > 
> > > > > > Signed-off-by: Christian König <christian.koenig@amd.com>
> > > > > > ---
> > > > > >    drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 66
> > > > > > ++++++++++++++++++++-
> > > > > >    drivers/gpu/drm/amd/amdgpu/amdgpu_object.c  |  6 ++
> > > > > >    2 files changed, 71 insertions(+), 1 deletion(-)
> > > > > > 
> > > > > > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> > > > > > b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> > > > > > index 770baba621b3..48de7624d49c 100644
> > > > > > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> > > > > > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> > > > > > @@ -453,7 +453,71 @@ amdgpu_dma_buf_create_obj(struct
> > > > > > drm_device *dev, struct dma_buf *dma_buf)
> > > > > >       return ERR_PTR(ret);
> > > > > >    }
> > > > > > 
> > > > > > +/**
> > > > > > + * amdgpu_dma_buf_move_notify - &attach.move_notify implementation
> > > > > > + *
> > > > > > + * @attach: the DMA-buf attachment
> > > > > > + *
> > > > > > + * Invalidate the DMA-buf attachment, making sure that
> > > > > > the we re-create the
> > > > > > + * mapping before the next use.
> > > > > > + */
> > > > > > +static void
> > > > > > +amdgpu_dma_buf_move_notify(struct dma_buf_attachment *attach)
> > > > > > +{
> > > > > > +    struct drm_gem_object *obj = attach->importer_priv;
> > > > > > +    struct ww_acquire_ctx *ticket = dma_resv_locking_ctx(obj->resv);
> > > > > > +    struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
> > > > > > +    struct amdgpu_device *adev = amdgpu_ttm_adev(bo->tbo.bdev);
> > > > > > +    struct ttm_operation_ctx ctx = { false, false };
> > > > > > +    struct ttm_placement placement = {};
> > > > > > +    struct amdgpu_vm_bo_base *bo_base;
> > > > > > +    int r;
> > > > > > +
> > > > > > +    if (bo->tbo.mem.mem_type == TTM_PL_SYSTEM)
> > > > > > +            return;
> > > > > > +
> > > > > > +    r = ttm_bo_validate(&bo->tbo, &placement, &ctx);
> > > > > > +    if (r) {
> > > > > > +            DRM_ERROR("Failed to invalidate DMA-buf
> > > > > > import (%d))\n", r);
> > > > > > +            return;
> > > > > > +    }
> > > > > > +
> > > > > > +    for (bo_base = bo->vm_bo; bo_base; bo_base = bo_base->next) {
> > > > > > +            struct amdgpu_vm *vm = bo_base->vm;
> > > > > > +            struct dma_resv *resv = vm->root.base.bo->tbo.base.resv;
> > > > > > +
> > > > > > +            if (ticket) {
> > > > > Yeah so this is kinda why I've been a total pain about the
> > > > > exact semantics
> > > > > of the move_notify hook. I think we should flat-out require
> > > > > that importers
> > > > > _always_ have a ticket attach when they call this, and that
> > > > > they can cope
> > > > > with additional locks being taken (i.e. full EDEADLCK) handling.
> > > > > 
> > > > > Simplest way to force that contract is to add a dummy 2nd
> > > > > ww_mutex lock to
> > > > > the dma_resv object, which we then can take #ifdef
> > > > > CONFIG_WW_MUTEX_SLOWPATH_DEBUG. Plus mabye a WARN_ON(!ticket).
> > > > > 
> > > > > Now the real disaster is how we handle deadlocks. Two issues:
> > > > > 
> > > > > - Ideally we'd keep any lock we've taken locked until the
> > > > > end, it helps
> > > > >     needless backoffs. I've played around a bit with that
> > > > > but not even poc
> > > > >     level, just an idea:
> > > > > 
> > > > > https://cgit.freedesktop.org/~danvet/drm/commit/?id=b1799c5a0f02df9e1bb08d27be37331255ab7582
> > > > > 
> > > > > 
> > > > >     Idea is essentially to track a list of objects we had to
> > > > > lock as part of
> > > > >     the ttm_bo_validate of the main object.
> > > > > 
> > > > > - Second one is if we get a EDEADLCK on one of these
> > > > > sublocks (like the
> > > > >     one here). We need to pass that up the entire callchain,
> > > > > including a
> > > > >     temporary reference (we have to drop locks to do the
> > > > > ww_mutex_lock_slow
> > > > >     call), and need a custom callback to drop that temporary reference
> > > > >     (since that's all driver specific, might even be
> > > > > internal ww_mutex and
> > > > >     not anything remotely looking like a normal dma_buf).
> > > > > This probably
> > > > >     needs the exec util helpers from ttm, but at the
> > > > > dma_resv level, so that
> > > > >     we can do something like this:
> > > > > 
> > > > > struct dma_resv_ticket {
> > > > >        struct ww_acquire_ctx base;
> > > > > 
> > > > >        /* can be set by anyone (including other drivers)
> > > > > that got hold of
> > > > >         * this ticket and had to acquire some new lock. This
> > > > > lock might
> > > > >         * protect anything, including driver-internal stuff, and isn't
> > > > >         * required to be a dma_buf or even just a dma_resv. */
> > > > >        struct ww_mutex *contended_lock;
> > > > > 
> > > > >        /* callback which the driver (which might be a dma-buf exporter
> > > > >         * and not matching the driver that started this
> > > > > locking ticket)
> > > > >         * sets together with @contended_lock, for the main
> > > > > driver to drop
> > > > >         * when it calls dma_resv_unlock on the contended_lock. */
> > > > >        void (drop_ref*)(struct ww_mutex *contended_lock);
> > > > > };
> > > > > 
> > > > > This is all supremely nasty (also ttm_bo_validate would need to be
> > > > > improved to handle these sublocks and random new objects
> > > > > that could force
> > > > > a ww_mutex_lock_slow).
> > > > > 
> > > > Just a short comment on this:
> > > > 
> > > > Neither the currently used wait-die or the wound-wait algorithm
> > > > *strictly* requires a slow lock on the contended lock. For
> > > > wait-die it's
> > > > just very convenient since it makes us sleep instead of spinning with
> > > > -EDEADLK on the contended lock. For wound-wait IIRC one could just
> > > > immediately restart the whole locking transaction after an
> > > > -EDEADLK, and
> > > > the transaction would automatically end up waiting on the contended
> > > > lock, provided the mutex lock stealing is not allowed. There is however
> > > > a possibility that the transaction will be wounded again on another
> > > > lock, taken before the contended lock, but I think there are ways to
> > > > improve the wound-wait algorithm to reduce that probability.
> > > > 
> > > > So in short, choosing the wound-wait algorithm instead of wait-die and
> > > > perhaps modifying the ww mutex code somewhat would probably help
> > > > passing
> > > > an -EDEADLK up the call chain without requiring passing the contended
> > > > lock, as long as each locker releases its own locks when receiving an
> > > > -EDEADLK.
> > > Hm this is kinda tempting, since rolling out the full backoff tricker
> > > across driver boundaries is going to be real painful.
> > > 
> > > What I'm kinda worried about is the debug/validation checks we're
> > > losing with this. The required backoff has this nice property that
> > > ww_mutex debug code can check that we've fully unwound everything when
> > > we should, that we've blocked on the right lock, and that we're
> > > restarting everything without keeling over. Without that I think we
> > > could end up with situations where a driver in the middle feels like
> > > handling the EDEADLCK, which might go well most of the times (the
> > > deadlock will probably be mostly within a given driver, not across).
> > > Right up to the point where someone creates a deadlock across drivers,
> > > and the lack of full rollback will be felt.
> > > 
> > > So not sure whether we can still keep all these debug/validation
> > > checks, or whether this is a step too far towards clever tricks.
> > 
> > I think we could definitely find a way to keep debugging to make sure
> > everything is unwound before attempting to restart the locking
> > transaction. But the debug check that we're restarting on the contended
> > lock only really makes sense for wait-die, (and we could easily keep it
> > for wait-die). The lock returning -EDEADLK for wound-wait may actually
> > not be the contending lock but an arbitrary lock that the wounded
> > transaction attempts to take after it is wounded.
> > 
> > So in the end IMO this is a tradeoff between added (possibly severe)
> > locking complexity into dma-buf and not being able to switch back to
> > wait-die efficiently if we need / want to do that.
> > 
> > /Thomas
> 
> And as a consequence an interface *could* be:
> 
> *) We introduce functions
> 
> void ww_acquire_relax(struct ww_acquire_ctx *ctx);
> int ww_acquire_relax_interruptible(struct ww_acquire_ctx *ctx);
> 
> that can be used instead of ww_mutex_lock_slow() in the absence of a
> contending lock to avoid spinning on -EDEADLK. While trying to take the
> contending lock is probably the best choice there are various second best
> approaches that can be explored, for example waiting on the contending
> acquire to finish or in the wound-wait case, perhaps do nothing. These
> functions will also help us keep the debugging.

Hm ... I guess this could work. Trouble is, it only gets rid of the
slowpath locking book-keeping headaches, we still have quite a few others.

> *) A function returning -EDEADLK to a caller *must* have already released
> its own locks.

So this ties to another question, as in should these callbacks have to
drops the locks thei acquire (much simpler code) or not (less thrashing,
if we drop locks we might end up in a situation where threads thrash
around instead of realizing quicker that they're actually deadlocking and
one of them should stop and back off).

But keeping locks locked means massive amounts of book-keeping in dma_resv
layer, so goes all downhill from there.

> *) move_notify() explicitly takes a struct ww_acquire_ctx * to make sure
> there is no ambiguity. (I think it would be valuable if we could do the same
> for ttm_bo_validate()).

Yeah I think more explicit locking ctx would be really good no matter
what. Implicitly fishing the acquire_ctx out of the lock for the object
you're called on is kinda nasty.
-Daniel
Thomas Hellström (VMware) Feb. 20, 2020, 7:46 p.m. UTC | #10
On 2/20/20 7:04 PM, Daniel Vetter wrote:
> On Thu, Feb 20, 2020 at 10:39:06AM +0100, Thomas Hellström (VMware) wrote:
>> On 2/19/20 7:42 AM, Thomas Hellström (VMware) wrote:
>>> On 2/18/20 10:01 PM, Daniel Vetter wrote:
>>>> On Tue, Feb 18, 2020 at 9:17 PM Thomas Hellström (VMware)
>>>> <thomas_os@shipmail.org> wrote:
>>>>> On 2/17/20 6:55 PM, Daniel Vetter wrote:
>>>>>> On Mon, Feb 17, 2020 at 04:45:09PM +0100, Christian König wrote:
>>>>>>> Implement the importer side of unpinned DMA-buf handling.
>>>>>>>
>>>>>>> v2: update page tables immediately
>>>>>>>
>>>>>>> Signed-off-by: Christian König <christian.koenig@amd.com>
>>>>>>> ---
>>>>>>>     drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 66
>>>>>>> ++++++++++++++++++++-
>>>>>>>     drivers/gpu/drm/amd/amdgpu/amdgpu_object.c  |  6 ++
>>>>>>>     2 files changed, 71 insertions(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>>>>> b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>>>>> index 770baba621b3..48de7624d49c 100644
>>>>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>>>>> @@ -453,7 +453,71 @@ amdgpu_dma_buf_create_obj(struct
>>>>>>> drm_device *dev, struct dma_buf *dma_buf)
>>>>>>>        return ERR_PTR(ret);
>>>>>>>     }
>>>>>>>
>>>>>>> +/**
>>>>>>> + * amdgpu_dma_buf_move_notify - &attach.move_notify implementation
>>>>>>> + *
>>>>>>> + * @attach: the DMA-buf attachment
>>>>>>> + *
>>>>>>> + * Invalidate the DMA-buf attachment, making sure that
>>>>>>> the we re-create the
>>>>>>> + * mapping before the next use.
>>>>>>> + */
>>>>>>> +static void
>>>>>>> +amdgpu_dma_buf_move_notify(struct dma_buf_attachment *attach)
>>>>>>> +{
>>>>>>> +    struct drm_gem_object *obj = attach->importer_priv;
>>>>>>> +    struct ww_acquire_ctx *ticket = dma_resv_locking_ctx(obj->resv);
>>>>>>> +    struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
>>>>>>> +    struct amdgpu_device *adev = amdgpu_ttm_adev(bo->tbo.bdev);
>>>>>>> +    struct ttm_operation_ctx ctx = { false, false };
>>>>>>> +    struct ttm_placement placement = {};
>>>>>>> +    struct amdgpu_vm_bo_base *bo_base;
>>>>>>> +    int r;
>>>>>>> +
>>>>>>> +    if (bo->tbo.mem.mem_type == TTM_PL_SYSTEM)
>>>>>>> +            return;
>>>>>>> +
>>>>>>> +    r = ttm_bo_validate(&bo->tbo, &placement, &ctx);
>>>>>>> +    if (r) {
>>>>>>> +            DRM_ERROR("Failed to invalidate DMA-buf
>>>>>>> import (%d))\n", r);
>>>>>>> +            return;
>>>>>>> +    }
>>>>>>> +
>>>>>>> +    for (bo_base = bo->vm_bo; bo_base; bo_base = bo_base->next) {
>>>>>>> +            struct amdgpu_vm *vm = bo_base->vm;
>>>>>>> +            struct dma_resv *resv = vm->root.base.bo->tbo.base.resv;
>>>>>>> +
>>>>>>> +            if (ticket) {
>>>>>> Yeah so this is kinda why I've been a total pain about the
>>>>>> exact semantics
>>>>>> of the move_notify hook. I think we should flat-out require
>>>>>> that importers
>>>>>> _always_ have a ticket attach when they call this, and that
>>>>>> they can cope
>>>>>> with additional locks being taken (i.e. full EDEADLCK) handling.
>>>>>>
>>>>>> Simplest way to force that contract is to add a dummy 2nd
>>>>>> ww_mutex lock to
>>>>>> the dma_resv object, which we then can take #ifdef
>>>>>> CONFIG_WW_MUTEX_SLOWPATH_DEBUG. Plus mabye a WARN_ON(!ticket).
>>>>>>
>>>>>> Now the real disaster is how we handle deadlocks. Two issues:
>>>>>>
>>>>>> - Ideally we'd keep any lock we've taken locked until the
>>>>>> end, it helps
>>>>>>      needless backoffs. I've played around a bit with that
>>>>>> but not even poc
>>>>>>      level, just an idea:
>>>>>>
>>>>>> https://cgit.freedesktop.org/~danvet/drm/commit/?id=b1799c5a0f02df9e1bb08d27be37331255ab7582
>>>>>>
>>>>>>
>>>>>>      Idea is essentially to track a list of objects we had to
>>>>>> lock as part of
>>>>>>      the ttm_bo_validate of the main object.
>>>>>>
>>>>>> - Second one is if we get a EDEADLCK on one of these
>>>>>> sublocks (like the
>>>>>>      one here). We need to pass that up the entire callchain,
>>>>>> including a
>>>>>>      temporary reference (we have to drop locks to do the
>>>>>> ww_mutex_lock_slow
>>>>>>      call), and need a custom callback to drop that temporary reference
>>>>>>      (since that's all driver specific, might even be
>>>>>> internal ww_mutex and
>>>>>>      not anything remotely looking like a normal dma_buf).
>>>>>> This probably
>>>>>>      needs the exec util helpers from ttm, but at the
>>>>>> dma_resv level, so that
>>>>>>      we can do something like this:
>>>>>>
>>>>>> struct dma_resv_ticket {
>>>>>>         struct ww_acquire_ctx base;
>>>>>>
>>>>>>         /* can be set by anyone (including other drivers)
>>>>>> that got hold of
>>>>>>          * this ticket and had to acquire some new lock. This
>>>>>> lock might
>>>>>>          * protect anything, including driver-internal stuff, and isn't
>>>>>>          * required to be a dma_buf or even just a dma_resv. */
>>>>>>         struct ww_mutex *contended_lock;
>>>>>>
>>>>>>         /* callback which the driver (which might be a dma-buf exporter
>>>>>>          * and not matching the driver that started this
>>>>>> locking ticket)
>>>>>>          * sets together with @contended_lock, for the main
>>>>>> driver to drop
>>>>>>          * when it calls dma_resv_unlock on the contended_lock. */
>>>>>>         void (drop_ref*)(struct ww_mutex *contended_lock);
>>>>>> };
>>>>>>
>>>>>> This is all supremely nasty (also ttm_bo_validate would need to be
>>>>>> improved to handle these sublocks and random new objects
>>>>>> that could force
>>>>>> a ww_mutex_lock_slow).
>>>>>>
>>>>> Just a short comment on this:
>>>>>
>>>>> Neither the currently used wait-die or the wound-wait algorithm
>>>>> *strictly* requires a slow lock on the contended lock. For
>>>>> wait-die it's
>>>>> just very convenient since it makes us sleep instead of spinning with
>>>>> -EDEADLK on the contended lock. For wound-wait IIRC one could just
>>>>> immediately restart the whole locking transaction after an
>>>>> -EDEADLK, and
>>>>> the transaction would automatically end up waiting on the contended
>>>>> lock, provided the mutex lock stealing is not allowed. There is however
>>>>> a possibility that the transaction will be wounded again on another
>>>>> lock, taken before the contended lock, but I think there are ways to
>>>>> improve the wound-wait algorithm to reduce that probability.
>>>>>
>>>>> So in short, choosing the wound-wait algorithm instead of wait-die and
>>>>> perhaps modifying the ww mutex code somewhat would probably help
>>>>> passing
>>>>> an -EDEADLK up the call chain without requiring passing the contended
>>>>> lock, as long as each locker releases its own locks when receiving an
>>>>> -EDEADLK.
>>>> Hm this is kinda tempting, since rolling out the full backoff tricker
>>>> across driver boundaries is going to be real painful.
>>>>
>>>> What I'm kinda worried about is the debug/validation checks we're
>>>> losing with this. The required backoff has this nice property that
>>>> ww_mutex debug code can check that we've fully unwound everything when
>>>> we should, that we've blocked on the right lock, and that we're
>>>> restarting everything without keeling over. Without that I think we
>>>> could end up with situations where a driver in the middle feels like
>>>> handling the EDEADLCK, which might go well most of the times (the
>>>> deadlock will probably be mostly within a given driver, not across).
>>>> Right up to the point where someone creates a deadlock across drivers,
>>>> and the lack of full rollback will be felt.
>>>>
>>>> So not sure whether we can still keep all these debug/validation
>>>> checks, or whether this is a step too far towards clever tricks.
>>> I think we could definitely find a way to keep debugging to make sure
>>> everything is unwound before attempting to restart the locking
>>> transaction. But the debug check that we're restarting on the contended
>>> lock only really makes sense for wait-die, (and we could easily keep it
>>> for wait-die). The lock returning -EDEADLK for wound-wait may actually
>>> not be the contending lock but an arbitrary lock that the wounded
>>> transaction attempts to take after it is wounded.
>>>
>>> So in the end IMO this is a tradeoff between added (possibly severe)
>>> locking complexity into dma-buf and not being able to switch back to
>>> wait-die efficiently if we need / want to do that.
>>>
>>> /Thomas
>> And as a consequence an interface *could* be:
>>
>> *) We introduce functions
>>
>> void ww_acquire_relax(struct ww_acquire_ctx *ctx);
>> int ww_acquire_relax_interruptible(struct ww_acquire_ctx *ctx);
>>
>> that can be used instead of ww_mutex_lock_slow() in the absence of a
>> contending lock to avoid spinning on -EDEADLK. While trying to take the
>> contending lock is probably the best choice there are various second best
>> approaches that can be explored, for example waiting on the contending
>> acquire to finish or in the wound-wait case, perhaps do nothing. These
>> functions will also help us keep the debugging.
> Hm ... I guess this could work. Trouble is, it only gets rid of the
> slowpath locking book-keeping headaches, we still have quite a few others.
>
>> *) A function returning -EDEADLK to a caller *must* have already released
>> its own locks.
> So this ties to another question, as in should these callbacks have to
> drops the locks thei acquire (much simpler code) or not (less thrashing,
> if we drop locks we might end up in a situation where threads thrash
> around instead of realizing quicker that they're actually deadlocking and
> one of them should stop and back off).

Hmm.. Could you describe such a thrashing case with an example?


/Thomas
Daniel Vetter Feb. 20, 2020, 8:08 p.m. UTC | #11
On Thu, Feb 20, 2020 at 08:46:27PM +0100, Thomas Hellström (VMware) wrote:
> On 2/20/20 7:04 PM, Daniel Vetter wrote:
> > On Thu, Feb 20, 2020 at 10:39:06AM +0100, Thomas Hellström (VMware) wrote:
> > > On 2/19/20 7:42 AM, Thomas Hellström (VMware) wrote:
> > > > On 2/18/20 10:01 PM, Daniel Vetter wrote:
> > > > > On Tue, Feb 18, 2020 at 9:17 PM Thomas Hellström (VMware)
> > > > > <thomas_os@shipmail.org> wrote:
> > > > > > On 2/17/20 6:55 PM, Daniel Vetter wrote:
> > > > > > > On Mon, Feb 17, 2020 at 04:45:09PM +0100, Christian König wrote:
> > > > > > > > Implement the importer side of unpinned DMA-buf handling.
> > > > > > > > 
> > > > > > > > v2: update page tables immediately
> > > > > > > > 
> > > > > > > > Signed-off-by: Christian König <christian.koenig@amd.com>
> > > > > > > > ---
> > > > > > > >     drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 66
> > > > > > > > ++++++++++++++++++++-
> > > > > > > >     drivers/gpu/drm/amd/amdgpu/amdgpu_object.c  |  6 ++
> > > > > > > >     2 files changed, 71 insertions(+), 1 deletion(-)
> > > > > > > > 
> > > > > > > > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> > > > > > > > b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> > > > > > > > index 770baba621b3..48de7624d49c 100644
> > > > > > > > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> > > > > > > > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> > > > > > > > @@ -453,7 +453,71 @@ amdgpu_dma_buf_create_obj(struct
> > > > > > > > drm_device *dev, struct dma_buf *dma_buf)
> > > > > > > >        return ERR_PTR(ret);
> > > > > > > >     }
> > > > > > > > 
> > > > > > > > +/**
> > > > > > > > + * amdgpu_dma_buf_move_notify - &attach.move_notify implementation
> > > > > > > > + *
> > > > > > > > + * @attach: the DMA-buf attachment
> > > > > > > > + *
> > > > > > > > + * Invalidate the DMA-buf attachment, making sure that
> > > > > > > > the we re-create the
> > > > > > > > + * mapping before the next use.
> > > > > > > > + */
> > > > > > > > +static void
> > > > > > > > +amdgpu_dma_buf_move_notify(struct dma_buf_attachment *attach)
> > > > > > > > +{
> > > > > > > > +    struct drm_gem_object *obj = attach->importer_priv;
> > > > > > > > +    struct ww_acquire_ctx *ticket = dma_resv_locking_ctx(obj->resv);
> > > > > > > > +    struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
> > > > > > > > +    struct amdgpu_device *adev = amdgpu_ttm_adev(bo->tbo.bdev);
> > > > > > > > +    struct ttm_operation_ctx ctx = { false, false };
> > > > > > > > +    struct ttm_placement placement = {};
> > > > > > > > +    struct amdgpu_vm_bo_base *bo_base;
> > > > > > > > +    int r;
> > > > > > > > +
> > > > > > > > +    if (bo->tbo.mem.mem_type == TTM_PL_SYSTEM)
> > > > > > > > +            return;
> > > > > > > > +
> > > > > > > > +    r = ttm_bo_validate(&bo->tbo, &placement, &ctx);
> > > > > > > > +    if (r) {
> > > > > > > > +            DRM_ERROR("Failed to invalidate DMA-buf
> > > > > > > > import (%d))\n", r);
> > > > > > > > +            return;
> > > > > > > > +    }
> > > > > > > > +
> > > > > > > > +    for (bo_base = bo->vm_bo; bo_base; bo_base = bo_base->next) {
> > > > > > > > +            struct amdgpu_vm *vm = bo_base->vm;
> > > > > > > > +            struct dma_resv *resv = vm->root.base.bo->tbo.base.resv;
> > > > > > > > +
> > > > > > > > +            if (ticket) {
> > > > > > > Yeah so this is kinda why I've been a total pain about the
> > > > > > > exact semantics
> > > > > > > of the move_notify hook. I think we should flat-out require
> > > > > > > that importers
> > > > > > > _always_ have a ticket attach when they call this, and that
> > > > > > > they can cope
> > > > > > > with additional locks being taken (i.e. full EDEADLCK) handling.
> > > > > > > 
> > > > > > > Simplest way to force that contract is to add a dummy 2nd
> > > > > > > ww_mutex lock to
> > > > > > > the dma_resv object, which we then can take #ifdef
> > > > > > > CONFIG_WW_MUTEX_SLOWPATH_DEBUG. Plus mabye a WARN_ON(!ticket).
> > > > > > > 
> > > > > > > Now the real disaster is how we handle deadlocks. Two issues:
> > > > > > > 
> > > > > > > - Ideally we'd keep any lock we've taken locked until the
> > > > > > > end, it helps
> > > > > > >      needless backoffs. I've played around a bit with that
> > > > > > > but not even poc
> > > > > > >      level, just an idea:
> > > > > > > 
> > > > > > > https://cgit.freedesktop.org/~danvet/drm/commit/?id=b1799c5a0f02df9e1bb08d27be37331255ab7582
> > > > > > > 
> > > > > > > 
> > > > > > >      Idea is essentially to track a list of objects we had to
> > > > > > > lock as part of
> > > > > > >      the ttm_bo_validate of the main object.
> > > > > > > 
> > > > > > > - Second one is if we get a EDEADLCK on one of these
> > > > > > > sublocks (like the
> > > > > > >      one here). We need to pass that up the entire callchain,
> > > > > > > including a
> > > > > > >      temporary reference (we have to drop locks to do the
> > > > > > > ww_mutex_lock_slow
> > > > > > >      call), and need a custom callback to drop that temporary reference
> > > > > > >      (since that's all driver specific, might even be
> > > > > > > internal ww_mutex and
> > > > > > >      not anything remotely looking like a normal dma_buf).
> > > > > > > This probably
> > > > > > >      needs the exec util helpers from ttm, but at the
> > > > > > > dma_resv level, so that
> > > > > > >      we can do something like this:
> > > > > > > 
> > > > > > > struct dma_resv_ticket {
> > > > > > >         struct ww_acquire_ctx base;
> > > > > > > 
> > > > > > >         /* can be set by anyone (including other drivers)
> > > > > > > that got hold of
> > > > > > >          * this ticket and had to acquire some new lock. This
> > > > > > > lock might
> > > > > > >          * protect anything, including driver-internal stuff, and isn't
> > > > > > >          * required to be a dma_buf or even just a dma_resv. */
> > > > > > >         struct ww_mutex *contended_lock;
> > > > > > > 
> > > > > > >         /* callback which the driver (which might be a dma-buf exporter
> > > > > > >          * and not matching the driver that started this
> > > > > > > locking ticket)
> > > > > > >          * sets together with @contended_lock, for the main
> > > > > > > driver to drop
> > > > > > >          * when it calls dma_resv_unlock on the contended_lock. */
> > > > > > >         void (drop_ref*)(struct ww_mutex *contended_lock);
> > > > > > > };
> > > > > > > 
> > > > > > > This is all supremely nasty (also ttm_bo_validate would need to be
> > > > > > > improved to handle these sublocks and random new objects
> > > > > > > that could force
> > > > > > > a ww_mutex_lock_slow).
> > > > > > > 
> > > > > > Just a short comment on this:
> > > > > > 
> > > > > > Neither the currently used wait-die or the wound-wait algorithm
> > > > > > *strictly* requires a slow lock on the contended lock. For
> > > > > > wait-die it's
> > > > > > just very convenient since it makes us sleep instead of spinning with
> > > > > > -EDEADLK on the contended lock. For wound-wait IIRC one could just
> > > > > > immediately restart the whole locking transaction after an
> > > > > > -EDEADLK, and
> > > > > > the transaction would automatically end up waiting on the contended
> > > > > > lock, provided the mutex lock stealing is not allowed. There is however
> > > > > > a possibility that the transaction will be wounded again on another
> > > > > > lock, taken before the contended lock, but I think there are ways to
> > > > > > improve the wound-wait algorithm to reduce that probability.
> > > > > > 
> > > > > > So in short, choosing the wound-wait algorithm instead of wait-die and
> > > > > > perhaps modifying the ww mutex code somewhat would probably help
> > > > > > passing
> > > > > > an -EDEADLK up the call chain without requiring passing the contended
> > > > > > lock, as long as each locker releases its own locks when receiving an
> > > > > > -EDEADLK.
> > > > > Hm this is kinda tempting, since rolling out the full backoff tricker
> > > > > across driver boundaries is going to be real painful.
> > > > > 
> > > > > What I'm kinda worried about is the debug/validation checks we're
> > > > > losing with this. The required backoff has this nice property that
> > > > > ww_mutex debug code can check that we've fully unwound everything when
> > > > > we should, that we've blocked on the right lock, and that we're
> > > > > restarting everything without keeling over. Without that I think we
> > > > > could end up with situations where a driver in the middle feels like
> > > > > handling the EDEADLCK, which might go well most of the times (the
> > > > > deadlock will probably be mostly within a given driver, not across).
> > > > > Right up to the point where someone creates a deadlock across drivers,
> > > > > and the lack of full rollback will be felt.
> > > > > 
> > > > > So not sure whether we can still keep all these debug/validation
> > > > > checks, or whether this is a step too far towards clever tricks.
> > > > I think we could definitely find a way to keep debugging to make sure
> > > > everything is unwound before attempting to restart the locking
> > > > transaction. But the debug check that we're restarting on the contended
> > > > lock only really makes sense for wait-die, (and we could easily keep it
> > > > for wait-die). The lock returning -EDEADLK for wound-wait may actually
> > > > not be the contending lock but an arbitrary lock that the wounded
> > > > transaction attempts to take after it is wounded.
> > > > 
> > > > So in the end IMO this is a tradeoff between added (possibly severe)
> > > > locking complexity into dma-buf and not being able to switch back to
> > > > wait-die efficiently if we need / want to do that.
> > > > 
> > > > /Thomas
> > > And as a consequence an interface *could* be:
> > > 
> > > *) We introduce functions
> > > 
> > > void ww_acquire_relax(struct ww_acquire_ctx *ctx);
> > > int ww_acquire_relax_interruptible(struct ww_acquire_ctx *ctx);
> > > 
> > > that can be used instead of ww_mutex_lock_slow() in the absence of a
> > > contending lock to avoid spinning on -EDEADLK. While trying to take the
> > > contending lock is probably the best choice there are various second best
> > > approaches that can be explored, for example waiting on the contending
> > > acquire to finish or in the wound-wait case, perhaps do nothing. These
> > > functions will also help us keep the debugging.
> > Hm ... I guess this could work. Trouble is, it only gets rid of the
> > slowpath locking book-keeping headaches, we still have quite a few others.
> > 
> > > *) A function returning -EDEADLK to a caller *must* have already released
> > > its own locks.
> > So this ties to another question, as in should these callbacks have to
> > drops the locks thei acquire (much simpler code) or not (less thrashing,
> > if we drop locks we might end up in a situation where threads thrash
> > around instead of realizing quicker that they're actually deadlocking and
> > one of them should stop and back off).
> 
> Hmm.. Could you describe such a thrashing case with an example?

Ignoring cross device fun and all that, just a simplified example of why
holding onto locks you've acquired for eviction is useful, at least in a
slow path.

- one thread trying to do an execbuf with a huge bo

vs.

- an entire pile of thread that try to do execbuf with just a few small bo

First thread is in the eviction loop, selects a bo, wins against all the
other thread since it's been doing this forever already, gets the bo moved
out, unlocks.

Since it's competing against lots of other threads with small bo, it'll
have to do that a lot of times. Often enough to create a contiguous hole.
If you have a smarter allocator that tries to create that hole more
actively, just assume that the single huge bo is a substantial part of
total vram.

The other threads will be quicker in cramming new stuff in, even if they
occasionally lose the ww dance against the single thread. So the big
thread livelocks.

If otoh the big thread would keep onto all the locks, eventually it have
the entire vram locked, and every other thread is guaranteed to lose
against it in the ww dance and queue up behind. And it could finally but
its huge bo into vram and execute.

Vary example for multi-gpu and more realism, but that's roughly it.

Aside, a lot of the stuff Christian has been doing in ttm is to improve
the chances that the competing threads will hit one of the locked objects
of the big thread, and at least back off a bit. That's at least my
understanding of what's been happening.
-Daniel
Thomas Hellström (VMware) Feb. 20, 2020, 10:51 p.m. UTC | #12
On 2/20/20 9:08 PM, Daniel Vetter wrote:
> On Thu, Feb 20, 2020 at 08:46:27PM +0100, Thomas Hellström (VMware) wrote:
>> On 2/20/20 7:04 PM, Daniel Vetter wrote:
>>> On Thu, Feb 20, 2020 at 10:39:06AM +0100, Thomas Hellström (VMware) wrote:
>>>> On 2/19/20 7:42 AM, Thomas Hellström (VMware) wrote:
>>>>> On 2/18/20 10:01 PM, Daniel Vetter wrote:
>>>>>> On Tue, Feb 18, 2020 at 9:17 PM Thomas Hellström (VMware)
>>>>>> <thomas_os@shipmail.org> wrote:
>>>>>>> On 2/17/20 6:55 PM, Daniel Vetter wrote:
>>>>>>>> On Mon, Feb 17, 2020 at 04:45:09PM +0100, Christian König wrote:
>>>>>>>>> Implement the importer side of unpinned DMA-buf handling.
>>>>>>>>>
>>>>>>>>> v2: update page tables immediately
>>>>>>>>>
>>>>>>>>> Signed-off-by: Christian König <christian.koenig@amd.com>
>>>>>>>>> ---
>>>>>>>>>      drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 66
>>>>>>>>> ++++++++++++++++++++-
>>>>>>>>>      drivers/gpu/drm/amd/amdgpu/amdgpu_object.c  |  6 ++
>>>>>>>>>      2 files changed, 71 insertions(+), 1 deletion(-)
>>>>>>>>>
>>>>>>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>>>>>>> b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>>>>>>> index 770baba621b3..48de7624d49c 100644
>>>>>>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>>>>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>>>>>>> @@ -453,7 +453,71 @@ amdgpu_dma_buf_create_obj(struct
>>>>>>>>> drm_device *dev, struct dma_buf *dma_buf)
>>>>>>>>>         return ERR_PTR(ret);
>>>>>>>>>      }
>>>>>>>>>
>>>>>>>>> +/**
>>>>>>>>> + * amdgpu_dma_buf_move_notify - &attach.move_notify implementation
>>>>>>>>> + *
>>>>>>>>> + * @attach: the DMA-buf attachment
>>>>>>>>> + *
>>>>>>>>> + * Invalidate the DMA-buf attachment, making sure that
>>>>>>>>> the we re-create the
>>>>>>>>> + * mapping before the next use.
>>>>>>>>> + */
>>>>>>>>> +static void
>>>>>>>>> +amdgpu_dma_buf_move_notify(struct dma_buf_attachment *attach)
>>>>>>>>> +{
>>>>>>>>> +    struct drm_gem_object *obj = attach->importer_priv;
>>>>>>>>> +    struct ww_acquire_ctx *ticket = dma_resv_locking_ctx(obj->resv);
>>>>>>>>> +    struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
>>>>>>>>> +    struct amdgpu_device *adev = amdgpu_ttm_adev(bo->tbo.bdev);
>>>>>>>>> +    struct ttm_operation_ctx ctx = { false, false };
>>>>>>>>> +    struct ttm_placement placement = {};
>>>>>>>>> +    struct amdgpu_vm_bo_base *bo_base;
>>>>>>>>> +    int r;
>>>>>>>>> +
>>>>>>>>> +    if (bo->tbo.mem.mem_type == TTM_PL_SYSTEM)
>>>>>>>>> +            return;
>>>>>>>>> +
>>>>>>>>> +    r = ttm_bo_validate(&bo->tbo, &placement, &ctx);
>>>>>>>>> +    if (r) {
>>>>>>>>> +            DRM_ERROR("Failed to invalidate DMA-buf
>>>>>>>>> import (%d))\n", r);
>>>>>>>>> +            return;
>>>>>>>>> +    }
>>>>>>>>> +
>>>>>>>>> +    for (bo_base = bo->vm_bo; bo_base; bo_base = bo_base->next) {
>>>>>>>>> +            struct amdgpu_vm *vm = bo_base->vm;
>>>>>>>>> +            struct dma_resv *resv = vm->root.base.bo->tbo.base.resv;
>>>>>>>>> +
>>>>>>>>> +            if (ticket) {
>>>>>>>> Yeah so this is kinda why I've been a total pain about the
>>>>>>>> exact semantics
>>>>>>>> of the move_notify hook. I think we should flat-out require
>>>>>>>> that importers
>>>>>>>> _always_ have a ticket attach when they call this, and that
>>>>>>>> they can cope
>>>>>>>> with additional locks being taken (i.e. full EDEADLCK) handling.
>>>>>>>>
>>>>>>>> Simplest way to force that contract is to add a dummy 2nd
>>>>>>>> ww_mutex lock to
>>>>>>>> the dma_resv object, which we then can take #ifdef
>>>>>>>> CONFIG_WW_MUTEX_SLOWPATH_DEBUG. Plus mabye a WARN_ON(!ticket).
>>>>>>>>
>>>>>>>> Now the real disaster is how we handle deadlocks. Two issues:
>>>>>>>>
>>>>>>>> - Ideally we'd keep any lock we've taken locked until the
>>>>>>>> end, it helps
>>>>>>>>       needless backoffs. I've played around a bit with that
>>>>>>>> but not even poc
>>>>>>>>       level, just an idea:
>>>>>>>>
>>>>>>>> https://cgit.freedesktop.org/~danvet/drm/commit/?id=b1799c5a0f02df9e1bb08d27be37331255ab7582
>>>>>>>>
>>>>>>>>
>>>>>>>>       Idea is essentially to track a list of objects we had to
>>>>>>>> lock as part of
>>>>>>>>       the ttm_bo_validate of the main object.
>>>>>>>>
>>>>>>>> - Second one is if we get a EDEADLCK on one of these
>>>>>>>> sublocks (like the
>>>>>>>>       one here). We need to pass that up the entire callchain,
>>>>>>>> including a
>>>>>>>>       temporary reference (we have to drop locks to do the
>>>>>>>> ww_mutex_lock_slow
>>>>>>>>       call), and need a custom callback to drop that temporary reference
>>>>>>>>       (since that's all driver specific, might even be
>>>>>>>> internal ww_mutex and
>>>>>>>>       not anything remotely looking like a normal dma_buf).
>>>>>>>> This probably
>>>>>>>>       needs the exec util helpers from ttm, but at the
>>>>>>>> dma_resv level, so that
>>>>>>>>       we can do something like this:
>>>>>>>>
>>>>>>>> struct dma_resv_ticket {
>>>>>>>>          struct ww_acquire_ctx base;
>>>>>>>>
>>>>>>>>          /* can be set by anyone (including other drivers)
>>>>>>>> that got hold of
>>>>>>>>           * this ticket and had to acquire some new lock. This
>>>>>>>> lock might
>>>>>>>>           * protect anything, including driver-internal stuff, and isn't
>>>>>>>>           * required to be a dma_buf or even just a dma_resv. */
>>>>>>>>          struct ww_mutex *contended_lock;
>>>>>>>>
>>>>>>>>          /* callback which the driver (which might be a dma-buf exporter
>>>>>>>>           * and not matching the driver that started this
>>>>>>>> locking ticket)
>>>>>>>>           * sets together with @contended_lock, for the main
>>>>>>>> driver to drop
>>>>>>>>           * when it calls dma_resv_unlock on the contended_lock. */
>>>>>>>>          void (drop_ref*)(struct ww_mutex *contended_lock);
>>>>>>>> };
>>>>>>>>
>>>>>>>> This is all supremely nasty (also ttm_bo_validate would need to be
>>>>>>>> improved to handle these sublocks and random new objects
>>>>>>>> that could force
>>>>>>>> a ww_mutex_lock_slow).
>>>>>>>>
>>>>>>> Just a short comment on this:
>>>>>>>
>>>>>>> Neither the currently used wait-die or the wound-wait algorithm
>>>>>>> *strictly* requires a slow lock on the contended lock. For
>>>>>>> wait-die it's
>>>>>>> just very convenient since it makes us sleep instead of spinning with
>>>>>>> -EDEADLK on the contended lock. For wound-wait IIRC one could just
>>>>>>> immediately restart the whole locking transaction after an
>>>>>>> -EDEADLK, and
>>>>>>> the transaction would automatically end up waiting on the contended
>>>>>>> lock, provided the mutex lock stealing is not allowed. There is however
>>>>>>> a possibility that the transaction will be wounded again on another
>>>>>>> lock, taken before the contended lock, but I think there are ways to
>>>>>>> improve the wound-wait algorithm to reduce that probability.
>>>>>>>
>>>>>>> So in short, choosing the wound-wait algorithm instead of wait-die and
>>>>>>> perhaps modifying the ww mutex code somewhat would probably help
>>>>>>> passing
>>>>>>> an -EDEADLK up the call chain without requiring passing the contended
>>>>>>> lock, as long as each locker releases its own locks when receiving an
>>>>>>> -EDEADLK.
>>>>>> Hm this is kinda tempting, since rolling out the full backoff tricker
>>>>>> across driver boundaries is going to be real painful.
>>>>>>
>>>>>> What I'm kinda worried about is the debug/validation checks we're
>>>>>> losing with this. The required backoff has this nice property that
>>>>>> ww_mutex debug code can check that we've fully unwound everything when
>>>>>> we should, that we've blocked on the right lock, and that we're
>>>>>> restarting everything without keeling over. Without that I think we
>>>>>> could end up with situations where a driver in the middle feels like
>>>>>> handling the EDEADLCK, which might go well most of the times (the
>>>>>> deadlock will probably be mostly within a given driver, not across).
>>>>>> Right up to the point where someone creates a deadlock across drivers,
>>>>>> and the lack of full rollback will be felt.
>>>>>>
>>>>>> So not sure whether we can still keep all these debug/validation
>>>>>> checks, or whether this is a step too far towards clever tricks.
>>>>> I think we could definitely find a way to keep debugging to make sure
>>>>> everything is unwound before attempting to restart the locking
>>>>> transaction. But the debug check that we're restarting on the contended
>>>>> lock only really makes sense for wait-die, (and we could easily keep it
>>>>> for wait-die). The lock returning -EDEADLK for wound-wait may actually
>>>>> not be the contending lock but an arbitrary lock that the wounded
>>>>> transaction attempts to take after it is wounded.
>>>>>
>>>>> So in the end IMO this is a tradeoff between added (possibly severe)
>>>>> locking complexity into dma-buf and not being able to switch back to
>>>>> wait-die efficiently if we need / want to do that.
>>>>>
>>>>> /Thomas
>>>> And as a consequence an interface *could* be:
>>>>
>>>> *) We introduce functions
>>>>
>>>> void ww_acquire_relax(struct ww_acquire_ctx *ctx);
>>>> int ww_acquire_relax_interruptible(struct ww_acquire_ctx *ctx);
>>>>
>>>> that can be used instead of ww_mutex_lock_slow() in the absence of a
>>>> contending lock to avoid spinning on -EDEADLK. While trying to take the
>>>> contending lock is probably the best choice there are various second best
>>>> approaches that can be explored, for example waiting on the contending
>>>> acquire to finish or in the wound-wait case, perhaps do nothing. These
>>>> functions will also help us keep the debugging.
>>> Hm ... I guess this could work. Trouble is, it only gets rid of the
>>> slowpath locking book-keeping headaches, we still have quite a few others.
>>>
>>>> *) A function returning -EDEADLK to a caller *must* have already released
>>>> its own locks.
>>> So this ties to another question, as in should these callbacks have to
>>> drops the locks thei acquire (much simpler code) or not (less thrashing,
>>> if we drop locks we might end up in a situation where threads thrash
>>> around instead of realizing quicker that they're actually deadlocking and
>>> one of them should stop and back off).
>> Hmm.. Could you describe such a thrashing case with an example?
> Ignoring cross device fun and all that, just a simplified example of why
> holding onto locks you've acquired for eviction is useful, at least in a
> slow path.
>
> - one thread trying to do an execbuf with a huge bo
>
> vs.
>
> - an entire pile of thread that try to do execbuf with just a few small bo
>
> First thread is in the eviction loop, selects a bo, wins against all the
> other thread since it's been doing this forever already, gets the bo moved
> out, unlocks.
>
> Since it's competing against lots of other threads with small bo, it'll
> have to do that a lot of times. Often enough to create a contiguous hole.
> If you have a smarter allocator that tries to create that hole more
> actively, just assume that the single huge bo is a substantial part of
> total vram.
>
> The other threads will be quicker in cramming new stuff in, even if they
> occasionally lose the ww dance against the single thread. So the big
> thread livelocks.
>
> If otoh the big thread would keep onto all the locks, eventually it have
> the entire vram locked, and every other thread is guaranteed to lose
> against it in the ww dance and queue up behind. And it could finally but
> its huge bo into vram and execute.

Hmm, yes this indeed explains why it's beneficial in some cases to keep 
a number of  locks held across certain operations, but I still fail to 
see why we would like *all* locks held across the entire transaction? In 
the above case I'd envision us ending up with something like:

int validate(ctx, bo)
{

     for_each_suitable_bo_to_evict(ebo) {
         r = lock(ctx, ebo);
         if (r == EDEADLK)
             goto out_unlock;

         r = move_notify(ctx, ebo);// locks and unlocks GPU VM bo.
         if (r == EDEADLK)
             goto out_unlock;
         evict();
     }

     place_bo(bo);
     //Repeat until success.


out_unlock:
     for_each_locked_bo(ebo)
         unlock(ctx, ebo);
}


void command_submission()
{
     acquire_init(ctx);

restart:
     for_each_bo_in_cs(bo) {
         r = lock(ctx, bo);
         if (r == -EDEADLK)
             goto out_unreserve;
     }

     for_each_bo_in_cs(bo) {
         r = validate(ctx, bo);
         if (r == -EDEADLK)
             goto out_unreserve;
     };

     cs();

     for_each_bo_in_cs(bo)
         unlock(ctx, bo);

     acquire_fini(ctx);
     return 0;

out_unreserve:
     for_each_locked_bo()
         unlock(ctx, bo);

     acquire_relax();
     goto restart;
}


>
> Vary example for multi-gpu and more realism, but that's roughly it.
>
> Aside, a lot of the stuff Christian has been doing in ttm is to improve
> the chances that the competing threads will hit one of the locked objects
> of the big thread, and at least back off a bit. That's at least my
> understanding of what's been happening.
> -Daniel

OK unserstood. For vmwgfx the crude simplistic idea to avoid that 
situation has been to have an rwsem around command submission: When the 
thread with the big bo has run a full LRU worth of eviction without 
succeeding it would get annoyed and take the rwsem in write mode, 
blocking competing threads. But that would of course never work in a 
dma-buf setting, and IIRC the implementation is not complete either....

/Thomas
Daniel Vetter Feb. 21, 2020, 5:12 p.m. UTC | #13
On Thu, Feb 20, 2020 at 11:51:07PM +0100, Thomas Hellström (VMware) wrote:
> On 2/20/20 9:08 PM, Daniel Vetter wrote:
> > On Thu, Feb 20, 2020 at 08:46:27PM +0100, Thomas Hellström (VMware) wrote:
> > > On 2/20/20 7:04 PM, Daniel Vetter wrote:
> > > > On Thu, Feb 20, 2020 at 10:39:06AM +0100, Thomas Hellström (VMware) wrote:
> > > > > On 2/19/20 7:42 AM, Thomas Hellström (VMware) wrote:
> > > > > > On 2/18/20 10:01 PM, Daniel Vetter wrote:
> > > > > > > On Tue, Feb 18, 2020 at 9:17 PM Thomas Hellström (VMware)
> > > > > > > <thomas_os@shipmail.org> wrote:
> > > > > > > > On 2/17/20 6:55 PM, Daniel Vetter wrote:
> > > > > > > > > On Mon, Feb 17, 2020 at 04:45:09PM +0100, Christian König wrote:
> > > > > > > > > > Implement the importer side of unpinned DMA-buf handling.
> > > > > > > > > > 
> > > > > > > > > > v2: update page tables immediately
> > > > > > > > > > 
> > > > > > > > > > Signed-off-by: Christian König <christian.koenig@amd.com>
> > > > > > > > > > ---
> > > > > > > > > >      drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 66
> > > > > > > > > > ++++++++++++++++++++-
> > > > > > > > > >      drivers/gpu/drm/amd/amdgpu/amdgpu_object.c  |  6 ++
> > > > > > > > > >      2 files changed, 71 insertions(+), 1 deletion(-)
> > > > > > > > > > 
> > > > > > > > > > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> > > > > > > > > > b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> > > > > > > > > > index 770baba621b3..48de7624d49c 100644
> > > > > > > > > > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> > > > > > > > > > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> > > > > > > > > > @@ -453,7 +453,71 @@ amdgpu_dma_buf_create_obj(struct
> > > > > > > > > > drm_device *dev, struct dma_buf *dma_buf)
> > > > > > > > > >         return ERR_PTR(ret);
> > > > > > > > > >      }
> > > > > > > > > > 
> > > > > > > > > > +/**
> > > > > > > > > > + * amdgpu_dma_buf_move_notify - &attach.move_notify implementation
> > > > > > > > > > + *
> > > > > > > > > > + * @attach: the DMA-buf attachment
> > > > > > > > > > + *
> > > > > > > > > > + * Invalidate the DMA-buf attachment, making sure that
> > > > > > > > > > the we re-create the
> > > > > > > > > > + * mapping before the next use.
> > > > > > > > > > + */
> > > > > > > > > > +static void
> > > > > > > > > > +amdgpu_dma_buf_move_notify(struct dma_buf_attachment *attach)
> > > > > > > > > > +{
> > > > > > > > > > +    struct drm_gem_object *obj = attach->importer_priv;
> > > > > > > > > > +    struct ww_acquire_ctx *ticket = dma_resv_locking_ctx(obj->resv);
> > > > > > > > > > +    struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
> > > > > > > > > > +    struct amdgpu_device *adev = amdgpu_ttm_adev(bo->tbo.bdev);
> > > > > > > > > > +    struct ttm_operation_ctx ctx = { false, false };
> > > > > > > > > > +    struct ttm_placement placement = {};
> > > > > > > > > > +    struct amdgpu_vm_bo_base *bo_base;
> > > > > > > > > > +    int r;
> > > > > > > > > > +
> > > > > > > > > > +    if (bo->tbo.mem.mem_type == TTM_PL_SYSTEM)
> > > > > > > > > > +            return;
> > > > > > > > > > +
> > > > > > > > > > +    r = ttm_bo_validate(&bo->tbo, &placement, &ctx);
> > > > > > > > > > +    if (r) {
> > > > > > > > > > +            DRM_ERROR("Failed to invalidate DMA-buf
> > > > > > > > > > import (%d))\n", r);
> > > > > > > > > > +            return;
> > > > > > > > > > +    }
> > > > > > > > > > +
> > > > > > > > > > +    for (bo_base = bo->vm_bo; bo_base; bo_base = bo_base->next) {
> > > > > > > > > > +            struct amdgpu_vm *vm = bo_base->vm;
> > > > > > > > > > +            struct dma_resv *resv = vm->root.base.bo->tbo.base.resv;
> > > > > > > > > > +
> > > > > > > > > > +            if (ticket) {
> > > > > > > > > Yeah so this is kinda why I've been a total pain about the
> > > > > > > > > exact semantics
> > > > > > > > > of the move_notify hook. I think we should flat-out require
> > > > > > > > > that importers
> > > > > > > > > _always_ have a ticket attach when they call this, and that
> > > > > > > > > they can cope
> > > > > > > > > with additional locks being taken (i.e. full EDEADLCK) handling.
> > > > > > > > > 
> > > > > > > > > Simplest way to force that contract is to add a dummy 2nd
> > > > > > > > > ww_mutex lock to
> > > > > > > > > the dma_resv object, which we then can take #ifdef
> > > > > > > > > CONFIG_WW_MUTEX_SLOWPATH_DEBUG. Plus mabye a WARN_ON(!ticket).
> > > > > > > > > 
> > > > > > > > > Now the real disaster is how we handle deadlocks. Two issues:
> > > > > > > > > 
> > > > > > > > > - Ideally we'd keep any lock we've taken locked until the
> > > > > > > > > end, it helps
> > > > > > > > >       needless backoffs. I've played around a bit with that
> > > > > > > > > but not even poc
> > > > > > > > >       level, just an idea:
> > > > > > > > > 
> > > > > > > > > https://cgit.freedesktop.org/~danvet/drm/commit/?id=b1799c5a0f02df9e1bb08d27be37331255ab7582
> > > > > > > > > 
> > > > > > > > > 
> > > > > > > > >       Idea is essentially to track a list of objects we had to
> > > > > > > > > lock as part of
> > > > > > > > >       the ttm_bo_validate of the main object.
> > > > > > > > > 
> > > > > > > > > - Second one is if we get a EDEADLCK on one of these
> > > > > > > > > sublocks (like the
> > > > > > > > >       one here). We need to pass that up the entire callchain,
> > > > > > > > > including a
> > > > > > > > >       temporary reference (we have to drop locks to do the
> > > > > > > > > ww_mutex_lock_slow
> > > > > > > > >       call), and need a custom callback to drop that temporary reference
> > > > > > > > >       (since that's all driver specific, might even be
> > > > > > > > > internal ww_mutex and
> > > > > > > > >       not anything remotely looking like a normal dma_buf).
> > > > > > > > > This probably
> > > > > > > > >       needs the exec util helpers from ttm, but at the
> > > > > > > > > dma_resv level, so that
> > > > > > > > >       we can do something like this:
> > > > > > > > > 
> > > > > > > > > struct dma_resv_ticket {
> > > > > > > > >          struct ww_acquire_ctx base;
> > > > > > > > > 
> > > > > > > > >          /* can be set by anyone (including other drivers)
> > > > > > > > > that got hold of
> > > > > > > > >           * this ticket and had to acquire some new lock. This
> > > > > > > > > lock might
> > > > > > > > >           * protect anything, including driver-internal stuff, and isn't
> > > > > > > > >           * required to be a dma_buf or even just a dma_resv. */
> > > > > > > > >          struct ww_mutex *contended_lock;
> > > > > > > > > 
> > > > > > > > >          /* callback which the driver (which might be a dma-buf exporter
> > > > > > > > >           * and not matching the driver that started this
> > > > > > > > > locking ticket)
> > > > > > > > >           * sets together with @contended_lock, for the main
> > > > > > > > > driver to drop
> > > > > > > > >           * when it calls dma_resv_unlock on the contended_lock. */
> > > > > > > > >          void (drop_ref*)(struct ww_mutex *contended_lock);
> > > > > > > > > };
> > > > > > > > > 
> > > > > > > > > This is all supremely nasty (also ttm_bo_validate would need to be
> > > > > > > > > improved to handle these sublocks and random new objects
> > > > > > > > > that could force
> > > > > > > > > a ww_mutex_lock_slow).
> > > > > > > > > 
> > > > > > > > Just a short comment on this:
> > > > > > > > 
> > > > > > > > Neither the currently used wait-die or the wound-wait algorithm
> > > > > > > > *strictly* requires a slow lock on the contended lock. For
> > > > > > > > wait-die it's
> > > > > > > > just very convenient since it makes us sleep instead of spinning with
> > > > > > > > -EDEADLK on the contended lock. For wound-wait IIRC one could just
> > > > > > > > immediately restart the whole locking transaction after an
> > > > > > > > -EDEADLK, and
> > > > > > > > the transaction would automatically end up waiting on the contended
> > > > > > > > lock, provided the mutex lock stealing is not allowed. There is however
> > > > > > > > a possibility that the transaction will be wounded again on another
> > > > > > > > lock, taken before the contended lock, but I think there are ways to
> > > > > > > > improve the wound-wait algorithm to reduce that probability.
> > > > > > > > 
> > > > > > > > So in short, choosing the wound-wait algorithm instead of wait-die and
> > > > > > > > perhaps modifying the ww mutex code somewhat would probably help
> > > > > > > > passing
> > > > > > > > an -EDEADLK up the call chain without requiring passing the contended
> > > > > > > > lock, as long as each locker releases its own locks when receiving an
> > > > > > > > -EDEADLK.
> > > > > > > Hm this is kinda tempting, since rolling out the full backoff tricker
> > > > > > > across driver boundaries is going to be real painful.
> > > > > > > 
> > > > > > > What I'm kinda worried about is the debug/validation checks we're
> > > > > > > losing with this. The required backoff has this nice property that
> > > > > > > ww_mutex debug code can check that we've fully unwound everything when
> > > > > > > we should, that we've blocked on the right lock, and that we're
> > > > > > > restarting everything without keeling over. Without that I think we
> > > > > > > could end up with situations where a driver in the middle feels like
> > > > > > > handling the EDEADLCK, which might go well most of the times (the
> > > > > > > deadlock will probably be mostly within a given driver, not across).
> > > > > > > Right up to the point where someone creates a deadlock across drivers,
> > > > > > > and the lack of full rollback will be felt.
> > > > > > > 
> > > > > > > So not sure whether we can still keep all these debug/validation
> > > > > > > checks, or whether this is a step too far towards clever tricks.
> > > > > > I think we could definitely find a way to keep debugging to make sure
> > > > > > everything is unwound before attempting to restart the locking
> > > > > > transaction. But the debug check that we're restarting on the contended
> > > > > > lock only really makes sense for wait-die, (and we could easily keep it
> > > > > > for wait-die). The lock returning -EDEADLK for wound-wait may actually
> > > > > > not be the contending lock but an arbitrary lock that the wounded
> > > > > > transaction attempts to take after it is wounded.
> > > > > > 
> > > > > > So in the end IMO this is a tradeoff between added (possibly severe)
> > > > > > locking complexity into dma-buf and not being able to switch back to
> > > > > > wait-die efficiently if we need / want to do that.
> > > > > > 
> > > > > > /Thomas
> > > > > And as a consequence an interface *could* be:
> > > > > 
> > > > > *) We introduce functions
> > > > > 
> > > > > void ww_acquire_relax(struct ww_acquire_ctx *ctx);
> > > > > int ww_acquire_relax_interruptible(struct ww_acquire_ctx *ctx);
> > > > > 
> > > > > that can be used instead of ww_mutex_lock_slow() in the absence of a
> > > > > contending lock to avoid spinning on -EDEADLK. While trying to take the
> > > > > contending lock is probably the best choice there are various second best
> > > > > approaches that can be explored, for example waiting on the contending
> > > > > acquire to finish or in the wound-wait case, perhaps do nothing. These
> > > > > functions will also help us keep the debugging.
> > > > Hm ... I guess this could work. Trouble is, it only gets rid of the
> > > > slowpath locking book-keeping headaches, we still have quite a few others.
> > > > 
> > > > > *) A function returning -EDEADLK to a caller *must* have already released
> > > > > its own locks.
> > > > So this ties to another question, as in should these callbacks have to
> > > > drops the locks thei acquire (much simpler code) or not (less thrashing,
> > > > if we drop locks we might end up in a situation where threads thrash
> > > > around instead of realizing quicker that they're actually deadlocking and
> > > > one of them should stop and back off).
> > > Hmm.. Could you describe such a thrashing case with an example?
> > Ignoring cross device fun and all that, just a simplified example of why
> > holding onto locks you've acquired for eviction is useful, at least in a
> > slow path.
> > 
> > - one thread trying to do an execbuf with a huge bo
> > 
> > vs.
> > 
> > - an entire pile of thread that try to do execbuf with just a few small bo
> > 
> > First thread is in the eviction loop, selects a bo, wins against all the
> > other thread since it's been doing this forever already, gets the bo moved
> > out, unlocks.
> > 
> > Since it's competing against lots of other threads with small bo, it'll
> > have to do that a lot of times. Often enough to create a contiguous hole.
> > If you have a smarter allocator that tries to create that hole more
> > actively, just assume that the single huge bo is a substantial part of
> > total vram.
> > 
> > The other threads will be quicker in cramming new stuff in, even if they
> > occasionally lose the ww dance against the single thread. So the big
> > thread livelocks.
> > 
> > If otoh the big thread would keep onto all the locks, eventually it have
> > the entire vram locked, and every other thread is guaranteed to lose
> > against it in the ww dance and queue up behind. And it could finally but
> > its huge bo into vram and execute.
> 
> Hmm, yes this indeed explains why it's beneficial in some cases to keep a
> number of  locks held across certain operations, but I still fail to see why
> we would like *all* locks held across the entire transaction? In the above
> case I'd envision us ending up with something like:
> 
> int validate(ctx, bo)
> {
> 
>     for_each_suitable_bo_to_evict(ebo) {
>         r = lock(ctx, ebo);
>         if (r == EDEADLK)
>             goto out_unlock;
> 
>         r = move_notify(ctx, ebo);// locks and unlocks GPU VM bo.

Yeah I think for move_notify the "keep the locks" thing is probably not
what we want. That's more for when you have to evict stuff and similar
things like that (which hopefully no driver needs to do in their
->move_notify). But for placing buffers we kinda want to keep things, and
that's also a cross-driver thing (eventually at least I think).

>         if (r == EDEADLK)
>             goto out_unlock;
>         evict();
>     }
> 
>     place_bo(bo);
>     //Repeat until success.
> 
> 
> out_unlock:
>     for_each_locked_bo(ebo)
>         unlock(ctx, ebo);

So that this unlock loop would need to be moved up to higher levels
perhaps. This here would solve the example of a single big bo, but if you
have multiple then you still end up with a lot of thrashing until the
younger thread realizes that it needs to back off.

> }
> 
> 
> void command_submission()
> {
>     acquire_init(ctx);
> 
> restart:
>     for_each_bo_in_cs(bo) {
>         r = lock(ctx, bo);
>         if (r == -EDEADLK)
>             goto out_unreserve;
>     }
> 
>     for_each_bo_in_cs(bo) {
>         r = validate(ctx, bo);
>         if (r == -EDEADLK)
>             goto out_unreserve;
>     };
> 
>     cs();
> 
>     for_each_bo_in_cs(bo)
>         unlock(ctx, bo);
> 
>     acquire_fini(ctx);
>     return 0;
> 
> out_unreserve:
>     for_each_locked_bo()
>         unlock(ctx, bo);
> 
>     acquire_relax();
>     goto restart;
> }
> > 
> > Vary example for multi-gpu and more realism, but that's roughly it.
> > 
> > Aside, a lot of the stuff Christian has been doing in ttm is to improve
> > the chances that the competing threads will hit one of the locked objects
> > of the big thread, and at least back off a bit. That's at least my
> > understanding of what's been happening.
> > -Daniel
> 
> OK unserstood. For vmwgfx the crude simplistic idea to avoid that situation
> has been to have an rwsem around command submission: When the thread with
> the big bo has run a full LRU worth of eviction without succeeding it would
> get annoyed and take the rwsem in write mode, blocking competing threads.
> But that would of course never work in a dma-buf setting, and IIRC the
> implementation is not complete either....

Yeah the Great Plan (tm) is to fully rely on ww_mutex slowly degenerating
into essentially a global lock. But only when there's actual contention
and thrashing.
-Daniel
Thomas Hellström (VMware) Feb. 21, 2020, 7:45 p.m. UTC | #14
On 2/21/20 6:12 PM, Daniel Vetter wrote:
> On Thu, Feb 20, 2020 at 11:51:07PM +0100, Thomas Hellström (VMware) wrote:
>> On 2/20/20 9:08 PM, Daniel Vetter wrote:
>>> On Thu, Feb 20, 2020 at 08:46:27PM +0100, Thomas Hellström (VMware) wrote:
>>>> On 2/20/20 7:04 PM, Daniel Vetter wrote:
>>>>> On Thu, Feb 20, 2020 at 10:39:06AM +0100, Thomas Hellström (VMware) wrote:
>>>>>> On 2/19/20 7:42 AM, Thomas Hellström (VMware) wrote:
>>>>>>> On 2/18/20 10:01 PM, Daniel Vetter wrote:
>>>>>>>> On Tue, Feb 18, 2020 at 9:17 PM Thomas Hellström (VMware)
>>>>>>>> <thomas_os@shipmail.org> wrote:
>>>>>>>>> On 2/17/20 6:55 PM, Daniel Vetter wrote:
>>>>>>>>>> On Mon, Feb 17, 2020 at 04:45:09PM +0100, Christian König wrote:
>>>>>>>>>>> Implement the importer side of unpinned DMA-buf handling.
>>>>>>>>>>>
>>>>>>>>>>> v2: update page tables immediately
>>>>>>>>>>>
>>>>>>>>>>> Signed-off-by: Christian König <christian.koenig@amd.com>
>>>>>>>>>>> ---
>>>>>>>>>>>       drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 66
>>>>>>>>>>> ++++++++++++++++++++-
>>>>>>>>>>>       drivers/gpu/drm/amd/amdgpu/amdgpu_object.c  |  6 ++
>>>>>>>>>>>       2 files changed, 71 insertions(+), 1 deletion(-)
>>>>>>>>>>>
>>>>>>>>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>>>>>>>>> b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>>>>>>>>> index 770baba621b3..48de7624d49c 100644
>>>>>>>>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>>>>>>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>>>>>>>>> @@ -453,7 +453,71 @@ amdgpu_dma_buf_create_obj(struct
>>>>>>>>>>> drm_device *dev, struct dma_buf *dma_buf)
>>>>>>>>>>>          return ERR_PTR(ret);
>>>>>>>>>>>       }
>>>>>>>>>>>
>>>>>>>>>>> +/**
>>>>>>>>>>> + * amdgpu_dma_buf_move_notify - &attach.move_notify implementation
>>>>>>>>>>> + *
>>>>>>>>>>> + * @attach: the DMA-buf attachment
>>>>>>>>>>> + *
>>>>>>>>>>> + * Invalidate the DMA-buf attachment, making sure that
>>>>>>>>>>> the we re-create the
>>>>>>>>>>> + * mapping before the next use.
>>>>>>>>>>> + */
>>>>>>>>>>> +static void
>>>>>>>>>>> +amdgpu_dma_buf_move_notify(struct dma_buf_attachment *attach)
>>>>>>>>>>> +{
>>>>>>>>>>> +    struct drm_gem_object *obj = attach->importer_priv;
>>>>>>>>>>> +    struct ww_acquire_ctx *ticket = dma_resv_locking_ctx(obj->resv);
>>>>>>>>>>> +    struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
>>>>>>>>>>> +    struct amdgpu_device *adev = amdgpu_ttm_adev(bo->tbo.bdev);
>>>>>>>>>>> +    struct ttm_operation_ctx ctx = { false, false };
>>>>>>>>>>> +    struct ttm_placement placement = {};
>>>>>>>>>>> +    struct amdgpu_vm_bo_base *bo_base;
>>>>>>>>>>> +    int r;
>>>>>>>>>>> +
>>>>>>>>>>> +    if (bo->tbo.mem.mem_type == TTM_PL_SYSTEM)
>>>>>>>>>>> +            return;
>>>>>>>>>>> +
>>>>>>>>>>> +    r = ttm_bo_validate(&bo->tbo, &placement, &ctx);
>>>>>>>>>>> +    if (r) {
>>>>>>>>>>> +            DRM_ERROR("Failed to invalidate DMA-buf
>>>>>>>>>>> import (%d))\n", r);
>>>>>>>>>>> +            return;
>>>>>>>>>>> +    }
>>>>>>>>>>> +
>>>>>>>>>>> +    for (bo_base = bo->vm_bo; bo_base; bo_base = bo_base->next) {
>>>>>>>>>>> +            struct amdgpu_vm *vm = bo_base->vm;
>>>>>>>>>>> +            struct dma_resv *resv = vm->root.base.bo->tbo.base.resv;
>>>>>>>>>>> +
>>>>>>>>>>> +            if (ticket) {
>>>>>>>>>> Yeah so this is kinda why I've been a total pain about the
>>>>>>>>>> exact semantics
>>>>>>>>>> of the move_notify hook. I think we should flat-out require
>>>>>>>>>> that importers
>>>>>>>>>> _always_ have a ticket attach when they call this, and that
>>>>>>>>>> they can cope
>>>>>>>>>> with additional locks being taken (i.e. full EDEADLCK) handling.
>>>>>>>>>>
>>>>>>>>>> Simplest way to force that contract is to add a dummy 2nd
>>>>>>>>>> ww_mutex lock to
>>>>>>>>>> the dma_resv object, which we then can take #ifdef
>>>>>>>>>> CONFIG_WW_MUTEX_SLOWPATH_DEBUG. Plus mabye a WARN_ON(!ticket).
>>>>>>>>>>
>>>>>>>>>> Now the real disaster is how we handle deadlocks. Two issues:
>>>>>>>>>>
>>>>>>>>>> - Ideally we'd keep any lock we've taken locked until the
>>>>>>>>>> end, it helps
>>>>>>>>>>        needless backoffs. I've played around a bit with that
>>>>>>>>>> but not even poc
>>>>>>>>>>        level, just an idea:
>>>>>>>>>>
>>>>>>>>>> https://cgit.freedesktop.org/~danvet/drm/commit/?id=b1799c5a0f02df9e1bb08d27be37331255ab7582
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>        Idea is essentially to track a list of objects we had to
>>>>>>>>>> lock as part of
>>>>>>>>>>        the ttm_bo_validate of the main object.
>>>>>>>>>>
>>>>>>>>>> - Second one is if we get a EDEADLCK on one of these
>>>>>>>>>> sublocks (like the
>>>>>>>>>>        one here). We need to pass that up the entire callchain,
>>>>>>>>>> including a
>>>>>>>>>>        temporary reference (we have to drop locks to do the
>>>>>>>>>> ww_mutex_lock_slow
>>>>>>>>>>        call), and need a custom callback to drop that temporary reference
>>>>>>>>>>        (since that's all driver specific, might even be
>>>>>>>>>> internal ww_mutex and
>>>>>>>>>>        not anything remotely looking like a normal dma_buf).
>>>>>>>>>> This probably
>>>>>>>>>>        needs the exec util helpers from ttm, but at the
>>>>>>>>>> dma_resv level, so that
>>>>>>>>>>        we can do something like this:
>>>>>>>>>>
>>>>>>>>>> struct dma_resv_ticket {
>>>>>>>>>>           struct ww_acquire_ctx base;
>>>>>>>>>>
>>>>>>>>>>           /* can be set by anyone (including other drivers)
>>>>>>>>>> that got hold of
>>>>>>>>>>            * this ticket and had to acquire some new lock. This
>>>>>>>>>> lock might
>>>>>>>>>>            * protect anything, including driver-internal stuff, and isn't
>>>>>>>>>>            * required to be a dma_buf or even just a dma_resv. */
>>>>>>>>>>           struct ww_mutex *contended_lock;
>>>>>>>>>>
>>>>>>>>>>           /* callback which the driver (which might be a dma-buf exporter
>>>>>>>>>>            * and not matching the driver that started this
>>>>>>>>>> locking ticket)
>>>>>>>>>>            * sets together with @contended_lock, for the main
>>>>>>>>>> driver to drop
>>>>>>>>>>            * when it calls dma_resv_unlock on the contended_lock. */
>>>>>>>>>>           void (drop_ref*)(struct ww_mutex *contended_lock);
>>>>>>>>>> };
>>>>>>>>>>
>>>>>>>>>> This is all supremely nasty (also ttm_bo_validate would need to be
>>>>>>>>>> improved to handle these sublocks and random new objects
>>>>>>>>>> that could force
>>>>>>>>>> a ww_mutex_lock_slow).
>>>>>>>>>>
>>>>>>>>> Just a short comment on this:
>>>>>>>>>
>>>>>>>>> Neither the currently used wait-die or the wound-wait algorithm
>>>>>>>>> *strictly* requires a slow lock on the contended lock. For
>>>>>>>>> wait-die it's
>>>>>>>>> just very convenient since it makes us sleep instead of spinning with
>>>>>>>>> -EDEADLK on the contended lock. For wound-wait IIRC one could just
>>>>>>>>> immediately restart the whole locking transaction after an
>>>>>>>>> -EDEADLK, and
>>>>>>>>> the transaction would automatically end up waiting on the contended
>>>>>>>>> lock, provided the mutex lock stealing is not allowed. There is however
>>>>>>>>> a possibility that the transaction will be wounded again on another
>>>>>>>>> lock, taken before the contended lock, but I think there are ways to
>>>>>>>>> improve the wound-wait algorithm to reduce that probability.
>>>>>>>>>
>>>>>>>>> So in short, choosing the wound-wait algorithm instead of wait-die and
>>>>>>>>> perhaps modifying the ww mutex code somewhat would probably help
>>>>>>>>> passing
>>>>>>>>> an -EDEADLK up the call chain without requiring passing the contended
>>>>>>>>> lock, as long as each locker releases its own locks when receiving an
>>>>>>>>> -EDEADLK.
>>>>>>>> Hm this is kinda tempting, since rolling out the full backoff tricker
>>>>>>>> across driver boundaries is going to be real painful.
>>>>>>>>
>>>>>>>> What I'm kinda worried about is the debug/validation checks we're
>>>>>>>> losing with this. The required backoff has this nice property that
>>>>>>>> ww_mutex debug code can check that we've fully unwound everything when
>>>>>>>> we should, that we've blocked on the right lock, and that we're
>>>>>>>> restarting everything without keeling over. Without that I think we
>>>>>>>> could end up with situations where a driver in the middle feels like
>>>>>>>> handling the EDEADLCK, which might go well most of the times (the
>>>>>>>> deadlock will probably be mostly within a given driver, not across).
>>>>>>>> Right up to the point where someone creates a deadlock across drivers,
>>>>>>>> and the lack of full rollback will be felt.
>>>>>>>>
>>>>>>>> So not sure whether we can still keep all these debug/validation
>>>>>>>> checks, or whether this is a step too far towards clever tricks.
>>>>>>> I think we could definitely find a way to keep debugging to make sure
>>>>>>> everything is unwound before attempting to restart the locking
>>>>>>> transaction. But the debug check that we're restarting on the contended
>>>>>>> lock only really makes sense for wait-die, (and we could easily keep it
>>>>>>> for wait-die). The lock returning -EDEADLK for wound-wait may actually
>>>>>>> not be the contending lock but an arbitrary lock that the wounded
>>>>>>> transaction attempts to take after it is wounded.
>>>>>>>
>>>>>>> So in the end IMO this is a tradeoff between added (possibly severe)
>>>>>>> locking complexity into dma-buf and not being able to switch back to
>>>>>>> wait-die efficiently if we need / want to do that.
>>>>>>>
>>>>>>> /Thomas
>>>>>> And as a consequence an interface *could* be:
>>>>>>
>>>>>> *) We introduce functions
>>>>>>
>>>>>> void ww_acquire_relax(struct ww_acquire_ctx *ctx);
>>>>>> int ww_acquire_relax_interruptible(struct ww_acquire_ctx *ctx);
>>>>>>
>>>>>> that can be used instead of ww_mutex_lock_slow() in the absence of a
>>>>>> contending lock to avoid spinning on -EDEADLK. While trying to take the
>>>>>> contending lock is probably the best choice there are various second best
>>>>>> approaches that can be explored, for example waiting on the contending
>>>>>> acquire to finish or in the wound-wait case, perhaps do nothing. These
>>>>>> functions will also help us keep the debugging.
>>>>> Hm ... I guess this could work. Trouble is, it only gets rid of the
>>>>> slowpath locking book-keeping headaches, we still have quite a few others.
>>>>>
>>>>>> *) A function returning -EDEADLK to a caller *must* have already released
>>>>>> its own locks.
>>>>> So this ties to another question, as in should these callbacks have to
>>>>> drops the locks thei acquire (much simpler code) or not (less thrashing,
>>>>> if we drop locks we might end up in a situation where threads thrash
>>>>> around instead of realizing quicker that they're actually deadlocking and
>>>>> one of them should stop and back off).
>>>> Hmm.. Could you describe such a thrashing case with an example?
>>> Ignoring cross device fun and all that, just a simplified example of why
>>> holding onto locks you've acquired for eviction is useful, at least in a
>>> slow path.
>>>
>>> - one thread trying to do an execbuf with a huge bo
>>>
>>> vs.
>>>
>>> - an entire pile of thread that try to do execbuf with just a few small bo
>>>
>>> First thread is in the eviction loop, selects a bo, wins against all the
>>> other thread since it's been doing this forever already, gets the bo moved
>>> out, unlocks.
>>>
>>> Since it's competing against lots of other threads with small bo, it'll
>>> have to do that a lot of times. Often enough to create a contiguous hole.
>>> If you have a smarter allocator that tries to create that hole more
>>> actively, just assume that the single huge bo is a substantial part of
>>> total vram.
>>>
>>> The other threads will be quicker in cramming new stuff in, even if they
>>> occasionally lose the ww dance against the single thread. So the big
>>> thread livelocks.
>>>
>>> If otoh the big thread would keep onto all the locks, eventually it have
>>> the entire vram locked, and every other thread is guaranteed to lose
>>> against it in the ww dance and queue up behind. And it could finally but
>>> its huge bo into vram and execute.
>> Hmm, yes this indeed explains why it's beneficial in some cases to keep a
>> number of  locks held across certain operations, but I still fail to see why
>> we would like *all* locks held across the entire transaction? In the above
>> case I'd envision us ending up with something like:
>>
>> int validate(ctx, bo)
>> {
>>
>>      for_each_suitable_bo_to_evict(ebo) {
>>          r = lock(ctx, ebo);
>>          if (r == EDEADLK)
>>              goto out_unlock;
>>
>>          r = move_notify(ctx, ebo);// locks and unlocks GPU VM bo.
> Yeah I think for move_notify the "keep the locks" thing is probably not
> what we want. That's more for when you have to evict stuff and similar
> things like that (which hopefully no driver needs to do in their
> ->move_notify). But for placing buffers we kinda want to keep things, and
> that's also a cross-driver thing (eventually at least I think).
>
>>          if (r == EDEADLK)
>>              goto out_unlock;
>>          evict();
>>      }
>>
>>      place_bo(bo);
>>      //Repeat until success.
>>
>>
>> out_unlock:
>>      for_each_locked_bo(ebo)
>>          unlock(ctx, ebo);
> So that this unlock loop would need to be moved up to higher levels
> perhaps. This here would solve the example of a single big bo, but if you
> have multiple then you still end up with a lot of thrashing until the
> younger thread realizes that it needs to back off.

Ah, so we hold on to resources we don't really need to attempt to block 
threads likely to be thrashing?

>
> Yeah the Great Plan (tm) is to fully rely on ww_mutex slowly degenerating
> into essentially a global lock. But only when there's actual contention
> and thrashing.

OK.

/Thomas



> -Daniel
Christian König Feb. 23, 2020, 3:45 p.m. UTC | #15
Am 21.02.20 um 18:12 schrieb Daniel Vetter:
> [SNIP]
> Yeah the Great Plan (tm) is to fully rely on ww_mutex slowly degenerating
> into essentially a global lock. But only when there's actual contention
> and thrashing.

Yes exactly. A really big problem in TTM is currently that we drop the 
lock after evicting BOs because they tend to move in again directly 
after that.

 From practice I can also confirm that there is exactly zero benefit 
from dropping locks early and reacquire them for example for the VM page 
tables. That's just makes it more likely that somebody needs to roll 
back and this is what we need to avoid in the first place.

Contention on BO locks during command submission is perfectly fine as 
long as this is as lightweight as possible while we don't have trashing. 
When we have trashing multi submission performance is best archived to 
just favor a single process to finish its business and block everybody else.

Because of this I would actually vote for forbidding to release 
individual ww_mutex() locks in a context.

Regards,
Christian.

> -Daniel
Thomas Hellström (VMware) Feb. 23, 2020, 4:54 p.m. UTC | #16
On 2/23/20 4:45 PM, Christian König wrote:
> Am 21.02.20 um 18:12 schrieb Daniel Vetter:
>> [SNIP]
>> Yeah the Great Plan (tm) is to fully rely on ww_mutex slowly 
>> degenerating
>> into essentially a global lock. But only when there's actual contention
>> and thrashing.
>
> Yes exactly. A really big problem in TTM is currently that we drop the 
> lock after evicting BOs because they tend to move in again directly 
> after that.
>
> From practice I can also confirm that there is exactly zero benefit 
> from dropping locks early and reacquire them for example for the VM 
> page tables. That's just makes it more likely that somebody needs to 
> roll back and this is what we need to avoid in the first place.


If you have a benchmarking setup available it would be very interesting 
for future reference to see how changing from WD to WW mutexes affects 
the roll back frequency. WW is known to cause rollbacks much less 
frequently but there is more work associated with each rollback.

>
> Contention on BO locks during command submission is perfectly fine as 
> long as this is as lightweight as possible while we don't have 
> trashing. When we have trashing multi submission performance is best 
> archived to just favor a single process to finish its business and 
> block everybody else.

Hmm. Sounds like we need a per-manager ww_rwsem protecting manager 
allocation, taken in write-mode then there's thrashing. In read-mode 
otherwise. That would limit the amount of "unnecessary" locks we'd have 
to keep and reduce unwanted side-effects, (see below):

>
> Because of this I would actually vote for forbidding to release 
> individual ww_mutex() locks in a context.

Yes, I see the problem.

But my first reaction is that this might have undersirable side-effects. 
Let's say somebody wanted to swap the evicted BOs out? Or cpu-writes to 
them causing faults, that might also block the mm_sem, which in turn 
blocks hugepaged?

Still it's a fairly simple solution to a problem that seems otherwise 
hard to solve efficiently.

Thanks,
Thomas


>
> Regards,
> Christian.
>
>> -Daniel
Christian König Feb. 24, 2020, 6:46 p.m. UTC | #17
Am 23.02.20 um 17:54 schrieb Thomas Hellström (VMware):
> On 2/23/20 4:45 PM, Christian König wrote:
>> Am 21.02.20 um 18:12 schrieb Daniel Vetter:
>>> [SNIP]
>>> Yeah the Great Plan (tm) is to fully rely on ww_mutex slowly 
>>> degenerating
>>> into essentially a global lock. But only when there's actual contention
>>> and thrashing.
>>
>> Yes exactly. A really big problem in TTM is currently that we drop 
>> the lock after evicting BOs because they tend to move in again 
>> directly after that.
>>
>> From practice I can also confirm that there is exactly zero benefit 
>> from dropping locks early and reacquire them for example for the VM 
>> page tables. That's just makes it more likely that somebody needs to 
>> roll back and this is what we need to avoid in the first place.
>
> If you have a benchmarking setup available it would be very 
> interesting for future reference to see how changing from WD to WW 
> mutexes affects the roll back frequency. WW is known to cause 
> rollbacks much less frequently but there is more work associated with 
> each rollback.

Not of hand. To be honest I still have a hard time to get a grip on the 
difference between WD and WW from the algorithm point of view. So I 
can't judge that difference at all.

>> Contention on BO locks during command submission is perfectly fine as 
>> long as this is as lightweight as possible while we don't have 
>> trashing. When we have trashing multi submission performance is best 
>> archived to just favor a single process to finish its business and 
>> block everybody else.
>
> Hmm. Sounds like we need a per-manager ww_rwsem protecting manager 
> allocation, taken in write-mode then there's thrashing. In read-mode 
> otherwise. That would limit the amount of "unnecessary" locks we'd 
> have to keep and reduce unwanted side-effects, (see below):

Well per-manager (you mean per domain here don't you?) doesn't sound 
like that useful because we rarely use only one domain, but I'm actually 
questioning for quite a while if the per BO lock scheme was the right 
approach.

See from the performance aspect the closest to ideal solution I can 
think of would be a ww_rwsem per user of a resource.

In other words we don't lock BOs, but instead a list of all their users 
and when you want to evict a BO you need to walk that list and inform 
all users that the BO will be moving.

During command submission you then have the fast path which rather just 
grabs the read side of the user lock and check if all BOs are still in 
the expected place.

If some BOs were evicted you back off and start the slow path, e.g. 
maybe even copy additional data from userspace then grab the write side 
of the lock etc.. etc...

That approach is similar to what we use in amdgpu with the per-VM BOs, 
but goes a step further. Problem is that we are so used to per BO locks 
in the kernel that this is probably not doable any more.

>> Because of this I would actually vote for forbidding to release 
>> individual ww_mutex() locks in a context.
>
> Yes, I see the problem.
>
> But my first reaction is that this might have undersirable 
> side-effects. Let's say somebody wanted to swap the evicted BOs out?

Please explain further, I off hand don't see the problem here.

In general I actually wanted to re-work TTM in a way that BOs in the 
SYSTEM/SWAPABLE domain are always backed by a shmem file instead of the 
struct page array we currently have.

> Or cpu-writes to them causing faults, that might also block the 
> mm_sem, which in turn blocks hugepaged?

Mhm, I also only have a higher level view how hugepaged works so why 
does it grabs the mm_sem on the write side?

Thanks,
Christian.

>
> Still it's a fairly simple solution to a problem that seems otherwise 
> hard to solve efficiently.
>
> Thanks,
> Thomas
>
>
>>
>> Regards,
>> Christian.
>>
>>> -Daniel
>
>
Thomas Hellström (VMware) Feb. 24, 2020, 9:11 p.m. UTC | #18
On 2/24/20 7:46 PM, Christian König wrote:
> Am 23.02.20 um 17:54 schrieb Thomas Hellström (VMware):
>> On 2/23/20 4:45 PM, Christian König wrote:
>>> Am 21.02.20 um 18:12 schrieb Daniel Vetter:
>>>> [SNIP]
>>>> Yeah the Great Plan (tm) is to fully rely on ww_mutex slowly 
>>>> degenerating
>>>> into essentially a global lock. But only when there's actual 
>>>> contention
>>>> and thrashing.
>>>
>>> Yes exactly. A really big problem in TTM is currently that we drop 
>>> the lock after evicting BOs because they tend to move in again 
>>> directly after that.
>>>
>>> From practice I can also confirm that there is exactly zero benefit 
>>> from dropping locks early and reacquire them for example for the VM 
>>> page tables. That's just makes it more likely that somebody needs to 
>>> roll back and this is what we need to avoid in the first place.
>>
>> If you have a benchmarking setup available it would be very 
>> interesting for future reference to see how changing from WD to WW 
>> mutexes affects the roll back frequency. WW is known to cause 
>> rollbacks much less frequently but there is more work associated with 
>> each rollback.
>
> Not of hand. To be honest I still have a hard time to get a grip on 
> the difference between WD and WW from the algorithm point of view. So 
> I can't judge that difference at all.

OK. I don't think a detailed understanding of the algorithms is strictly 
necessary, though. If we had had a good testcase we'd just have to 
change DEFINE_WD_CLASS in dma-resv.c to DEFINE_WW_CLASS and benchmark 
relevant figures.


>
>>> Contention on BO locks during command submission is perfectly fine 
>>> as long as this is as lightweight as possible while we don't have 
>>> trashing. When we have trashing multi submission performance is best 
>>> archived to just favor a single process to finish its business and 
>>> block everybody else.
>>
>> Hmm. Sounds like we need a per-manager ww_rwsem protecting manager 
>> allocation, taken in write-mode then there's thrashing. In read-mode 
>> otherwise. That would limit the amount of "unnecessary" locks we'd 
>> have to keep and reduce unwanted side-effects, (see below):
>
> Well per-manager (you mean per domain here don't you?)
yes.
> doesn't sound like that useful because we rarely use only one domain,

Well the difference to keeping all locks would boil down to:
"Instead of keeping all locks of all bos we evict from thrashing 
domains, keep locks of all thrashing domains in write mode". This means 
that for domains that are not thrashing, we'd just keep read locks.


> but I'm actually questioning for quite a while if the per BO lock 
> scheme was the right approach.
>
> See from the performance aspect the closest to ideal solution I can 
> think of would be a ww_rwsem per user of a resource.
>
> In other words we don't lock BOs, but instead a list of all their 
> users and when you want to evict a BO you need to walk that list and 
> inform all users that the BO will be moving.
>
> During command submission you then have the fast path which rather 
> just grabs the read side of the user lock and check if all BOs are 
> still in the expected place.
>
> If some BOs were evicted you back off and start the slow path, e.g. 
> maybe even copy additional data from userspace then grab the write 
> side of the lock etc.. etc...
>
> That approach is similar to what we use in amdgpu with the per-VM BOs, 
> but goes a step further. Problem is that we are so used to per BO 
> locks in the kernel that this is probably not doable any more.

I think we need some-sort of per-bo lock to protect bo metadata. But 
yes, relying solely on them to resolve other resource (domain) 
contention may not be (or rather probably isn't) the right choice.

>
>>> Because of this I would actually vote for forbidding to release 
>>> individual ww_mutex() locks in a context.
>>
>> Yes, I see the problem.
>>
>> But my first reaction is that this might have undersirable 
>> side-effects. Let's say somebody wanted to swap the evicted BOs out?
>
> Please explain further, I off hand don't see the problem here.

Lets say thread A evicts a lot of thread B's bos, and keeps the locks of 
those bos for a prolonged time. Then thread C needs memory and wants to 
swap out thread B's bos. It can't, or at least not during a certain 
delay because thread A unnecessarily holds the locks.

>
> In general I actually wanted to re-work TTM in a way that BOs in the 
> SYSTEM/SWAPABLE domain are always backed by a shmem file instead of 
> the struct page array we currently have.

That would probably work well if there are no SYSTEM+write-combined 
users anymore. Typically in the old AGP days, you wouldn't change 
caching mode when evicting from write-combine AGP to SYSTEM because of 
the dead slow wbinvd() operation.
>
>> Or cpu-writes to them causing faults, that might also block the 
>> mm_sem, which in turn blocks hugepaged?
>
> Mhm, I also only have a higher level view how hugepaged works so why 
> does it grabs the mm_sem on the write side?

If I understand it correctly, it's needed when collapsing PMD 
directories to huge PMD pages. But this was merely an example. For this 
particular case the RETRY mechanism in the TTM fault handler we've 
discussed before will try reasonably hard to release the mmap_sem when 
sleeping on a bo lock.

Thanks,
Thomas



>
> Thanks,
> Christian.
>
>>
>> Still it's a fairly simple solution to a problem that seems otherwise 
>> hard to solve efficiently.
>>
>> Thanks,
>> Thomas
>>
>>
>>>
>>> Regards,
>>> Christian.
>>>
>>>> -Daniel
>>
>>
Daniel Vetter Feb. 25, 2020, 5:16 p.m. UTC | #19
On Mon, Feb 24, 2020 at 07:46:59PM +0100, Christian König wrote:
> Am 23.02.20 um 17:54 schrieb Thomas Hellström (VMware):
> > On 2/23/20 4:45 PM, Christian König wrote:
> > > Am 21.02.20 um 18:12 schrieb Daniel Vetter:
> > > > [SNIP]
> > > > Yeah the Great Plan (tm) is to fully rely on ww_mutex slowly
> > > > degenerating
> > > > into essentially a global lock. But only when there's actual contention
> > > > and thrashing.
> > > 
> > > Yes exactly. A really big problem in TTM is currently that we drop
> > > the lock after evicting BOs because they tend to move in again
> > > directly after that.
> > > 
> > > From practice I can also confirm that there is exactly zero benefit
> > > from dropping locks early and reacquire them for example for the VM
> > > page tables. That's just makes it more likely that somebody needs to
> > > roll back and this is what we need to avoid in the first place.
> > 
> > If you have a benchmarking setup available it would be very interesting
> > for future reference to see how changing from WD to WW mutexes affects
> > the roll back frequency. WW is known to cause rollbacks much less
> > frequently but there is more work associated with each rollback.
> 
> Not of hand. To be honest I still have a hard time to get a grip on the
> difference between WD and WW from the algorithm point of view. So I can't
> judge that difference at all.
> 
> > > Contention on BO locks during command submission is perfectly fine
> > > as long as this is as lightweight as possible while we don't have
> > > trashing. When we have trashing multi submission performance is best
> > > archived to just favor a single process to finish its business and
> > > block everybody else.
> > 
> > Hmm. Sounds like we need a per-manager ww_rwsem protecting manager
> > allocation, taken in write-mode then there's thrashing. In read-mode
> > otherwise. That would limit the amount of "unnecessary" locks we'd have
> > to keep and reduce unwanted side-effects, (see below):
> 
> Well per-manager (you mean per domain here don't you?) doesn't sound like
> that useful because we rarely use only one domain, but I'm actually
> questioning for quite a while if the per BO lock scheme was the right
> approach.
> 
> See from the performance aspect the closest to ideal solution I can think of
> would be a ww_rwsem per user of a resource.
> 
> In other words we don't lock BOs, but instead a list of all their users and
> when you want to evict a BO you need to walk that list and inform all users
> that the BO will be moving.
> 
> During command submission you then have the fast path which rather just
> grabs the read side of the user lock and check if all BOs are still in the
> expected place.
> 
> If some BOs were evicted you back off and start the slow path, e.g. maybe
> even copy additional data from userspace then grab the write side of the
> lock etc.. etc...
> 
> That approach is similar to what we use in amdgpu with the per-VM BOs, but
> goes a step further. Problem is that we are so used to per BO locks in the
> kernel that this is probably not doable any more.

Yeah I think it'd be nice to have the same approach for shared bo too. I
guess what we could do is something like this (spinning your ww_rwmutex
idea a bit further):

dma_buf_read_lock(buf, vm)
{
	if (enabled(CONFIG_DEBUG_WW_MUTEX_SLOWPATH))
	{
		check that vm is indeed listed in buf and splat if not
	}

	/* for a buf that's not shared in multiple vm we'd have buf->resv
	 * == vm->resv here */
	return ww_mutex_lock(vm->resv);
}

dma_buf_write_lock(buf)
{
	for_each_vm_in_buf(buf, vm) {
		ww_mutex_lock(vm->resv);
	}
}

Ideally we'd track all these vms with something slightly less shoddy than
a linked list :-) Resizeable array is probably pretty good, I think we
only ever need to go from buf -> vm list, not the other way round. At
least in dma_resv/dma_buf code, driver code ofc needs to keep a list of
all bo bound to a vm somewhere. But that's probably a much bigger
datastructure for tracking vma offsets and mappings and other things on
top.

Ofc to even just get there we'd need something like the sublock list to
keep track of all the additional locks we'd need for the writer lock. And
we'd need the release callback for backoff, so that we could also go
through the slowpath on a vm object that we're not holding a full
reference on. That also means vm need to be refcounted.

And the list of vms on a buffer need to be protected with some lock and
the usual kref_get_unless_zero trickery.

But with all this I think we can make the dma_buf_write_lock lock 100%
like the old per-buffer lock for everyone. And execbuf could switch over
to dma_buf_read_lock for shared buffers. Bonus points when the gpu context
just keeps track of a list of shared vm used by buffers in that context
...

That way we could make vm fastpath locking a la amdgpu opt-in, while
keeping everyone else on the per-object locking juices.

Thoughts?

Cheers, Daniel

PS: Of course the write lock for these buffers is going to be terrible, so
every time you need to update fences for implicit sync on shared buffer
(well write fence at least) it's going to suck. We probably also want a
read_to_write_upgrade function, which also can be done easily with
ww_mutex magic.
Daniel Vetter Feb. 26, 2020, 4:32 p.m. UTC | #20
On Tue, Feb 25, 2020 at 6:16 PM Daniel Vetter <daniel@ffwll.ch> wrote:
>
> On Mon, Feb 24, 2020 at 07:46:59PM +0100, Christian König wrote:
> > Am 23.02.20 um 17:54 schrieb Thomas Hellström (VMware):
> > > On 2/23/20 4:45 PM, Christian König wrote:
> > > > Am 21.02.20 um 18:12 schrieb Daniel Vetter:
> > > > > [SNIP]
> > > > > Yeah the Great Plan (tm) is to fully rely on ww_mutex slowly
> > > > > degenerating
> > > > > into essentially a global lock. But only when there's actual contention
> > > > > and thrashing.
> > > >
> > > > Yes exactly. A really big problem in TTM is currently that we drop
> > > > the lock after evicting BOs because they tend to move in again
> > > > directly after that.
> > > >
> > > > From practice I can also confirm that there is exactly zero benefit
> > > > from dropping locks early and reacquire them for example for the VM
> > > > page tables. That's just makes it more likely that somebody needs to
> > > > roll back and this is what we need to avoid in the first place.
> > >
> > > If you have a benchmarking setup available it would be very interesting
> > > for future reference to see how changing from WD to WW mutexes affects
> > > the roll back frequency. WW is known to cause rollbacks much less
> > > frequently but there is more work associated with each rollback.
> >
> > Not of hand. To be honest I still have a hard time to get a grip on the
> > difference between WD and WW from the algorithm point of view. So I can't
> > judge that difference at all.
> >
> > > > Contention on BO locks during command submission is perfectly fine
> > > > as long as this is as lightweight as possible while we don't have
> > > > trashing. When we have trashing multi submission performance is best
> > > > archived to just favor a single process to finish its business and
> > > > block everybody else.
> > >
> > > Hmm. Sounds like we need a per-manager ww_rwsem protecting manager
> > > allocation, taken in write-mode then there's thrashing. In read-mode
> > > otherwise. That would limit the amount of "unnecessary" locks we'd have
> > > to keep and reduce unwanted side-effects, (see below):
> >
> > Well per-manager (you mean per domain here don't you?) doesn't sound like
> > that useful because we rarely use only one domain, but I'm actually
> > questioning for quite a while if the per BO lock scheme was the right
> > approach.
> >
> > See from the performance aspect the closest to ideal solution I can think of
> > would be a ww_rwsem per user of a resource.
> >
> > In other words we don't lock BOs, but instead a list of all their users and
> > when you want to evict a BO you need to walk that list and inform all users
> > that the BO will be moving.
> >
> > During command submission you then have the fast path which rather just
> > grabs the read side of the user lock and check if all BOs are still in the
> > expected place.
> >
> > If some BOs were evicted you back off and start the slow path, e.g. maybe
> > even copy additional data from userspace then grab the write side of the
> > lock etc.. etc...
> >
> > That approach is similar to what we use in amdgpu with the per-VM BOs, but
> > goes a step further. Problem is that we are so used to per BO locks in the
> > kernel that this is probably not doable any more.
>
> Yeah I think it'd be nice to have the same approach for shared bo too. I
> guess what we could do is something like this (spinning your ww_rwmutex
> idea a bit further):
>
> dma_buf_read_lock(buf, vm)
> {
>         if (enabled(CONFIG_DEBUG_WW_MUTEX_SLOWPATH))
>         {
>                 check that vm is indeed listed in buf and splat if not
>         }
>
>         /* for a buf that's not shared in multiple vm we'd have buf->resv
>          * == vm->resv here */
>         return ww_mutex_lock(vm->resv);
> }
>
> dma_buf_write_lock(buf)
> {
>         for_each_vm_in_buf(buf, vm) {
>                 ww_mutex_lock(vm->resv);
>         }
> }
>
> Ideally we'd track all these vms with something slightly less shoddy than
> a linked list :-) Resizeable array is probably pretty good, I think we
> only ever need to go from buf -> vm list, not the other way round. At
> least in dma_resv/dma_buf code, driver code ofc needs to keep a list of
> all bo bound to a vm somewhere. But that's probably a much bigger
> datastructure for tracking vma offsets and mappings and other things on
> top.
>
> Ofc to even just get there we'd need something like the sublock list to
> keep track of all the additional locks we'd need for the writer lock. And
> we'd need the release callback for backoff, so that we could also go
> through the slowpath on a vm object that we're not holding a full
> reference on. That also means vm need to be refcounted.
>
> And the list of vms on a buffer need to be protected with some lock and
> the usual kref_get_unless_zero trickery.
>
> But with all this I think we can make the dma_buf_write_lock lock 100%
> like the old per-buffer lock for everyone. And execbuf could switch over
> to dma_buf_read_lock for shared buffers. Bonus points when the gpu context
> just keeps track of a list of shared vm used by buffers in that context
> ...
>
> That way we could make vm fastpath locking a la amdgpu opt-in, while
> keeping everyone else on the per-object locking juices.
>
> Thoughts?

One thing I just realized, which is nasty: The full (write) lock needs
ww_acquire_ctx with this, because it needs to take a bunch of locks.
Rolling that out everywhere is going to be nasty.

I guess though we could do a fallback and have a locally created
ww_acquire_ctx if there's none passed in, with backoff entirely
implemented within dma_resv_lock.
-Daniel

>
> Cheers, Daniel
>
> PS: Of course the write lock for these buffers is going to be terrible, so
> every time you need to update fences for implicit sync on shared buffer
> (well write fence at least) it's going to suck. We probably also want a
> read_to_write_upgrade function, which also can be done easily with
> ww_mutex magic.
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
Christian König Feb. 27, 2020, 9:20 a.m. UTC | #21
Am 26.02.20 um 17:32 schrieb Daniel Vetter:
> On Tue, Feb 25, 2020 at 6:16 PM Daniel Vetter <daniel@ffwll.ch> wrote:
>> On Mon, Feb 24, 2020 at 07:46:59PM +0100, Christian König wrote:
>>> Am 23.02.20 um 17:54 schrieb Thomas Hellström (VMware):
>>>> On 2/23/20 4:45 PM, Christian König wrote:
>>>>> Am 21.02.20 um 18:12 schrieb Daniel Vetter:
>>>>>> [SNIP]
>>>>>> Yeah the Great Plan (tm) is to fully rely on ww_mutex slowly
>>>>>> degenerating
>>>>>> into essentially a global lock. But only when there's actual contention
>>>>>> and thrashing.
>>>>> Yes exactly. A really big problem in TTM is currently that we drop
>>>>> the lock after evicting BOs because they tend to move in again
>>>>> directly after that.
>>>>>
>>>>>  From practice I can also confirm that there is exactly zero benefit
>>>>> from dropping locks early and reacquire them for example for the VM
>>>>> page tables. That's just makes it more likely that somebody needs to
>>>>> roll back and this is what we need to avoid in the first place.
>>>> If you have a benchmarking setup available it would be very interesting
>>>> for future reference to see how changing from WD to WW mutexes affects
>>>> the roll back frequency. WW is known to cause rollbacks much less
>>>> frequently but there is more work associated with each rollback.
>>> Not of hand. To be honest I still have a hard time to get a grip on the
>>> difference between WD and WW from the algorithm point of view. So I can't
>>> judge that difference at all.
>>>
>>>>> Contention on BO locks during command submission is perfectly fine
>>>>> as long as this is as lightweight as possible while we don't have
>>>>> trashing. When we have trashing multi submission performance is best
>>>>> archived to just favor a single process to finish its business and
>>>>> block everybody else.
>>>> Hmm. Sounds like we need a per-manager ww_rwsem protecting manager
>>>> allocation, taken in write-mode then there's thrashing. In read-mode
>>>> otherwise. That would limit the amount of "unnecessary" locks we'd have
>>>> to keep and reduce unwanted side-effects, (see below):
>>> Well per-manager (you mean per domain here don't you?) doesn't sound like
>>> that useful because we rarely use only one domain, but I'm actually
>>> questioning for quite a while if the per BO lock scheme was the right
>>> approach.
>>>
>>> See from the performance aspect the closest to ideal solution I can think of
>>> would be a ww_rwsem per user of a resource.
>>>
>>> In other words we don't lock BOs, but instead a list of all their users and
>>> when you want to evict a BO you need to walk that list and inform all users
>>> that the BO will be moving.
>>>
>>> During command submission you then have the fast path which rather just
>>> grabs the read side of the user lock and check if all BOs are still in the
>>> expected place.
>>>
>>> If some BOs were evicted you back off and start the slow path, e.g. maybe
>>> even copy additional data from userspace then grab the write side of the
>>> lock etc.. etc...
>>>
>>> That approach is similar to what we use in amdgpu with the per-VM BOs, but
>>> goes a step further. Problem is that we are so used to per BO locks in the
>>> kernel that this is probably not doable any more.
>> Yeah I think it'd be nice to have the same approach for shared bo too. I
>> guess what we could do is something like this (spinning your ww_rwmutex
>> idea a bit further):
>>
>> dma_buf_read_lock(buf, vm)
>> {
>>          if (enabled(CONFIG_DEBUG_WW_MUTEX_SLOWPATH))
>>          {
>>                  check that vm is indeed listed in buf and splat if not
>>          }
>>
>>          /* for a buf that's not shared in multiple vm we'd have buf->resv
>>           * == vm->resv here */
>>          return ww_mutex_lock(vm->resv);
>> }
>>
>> dma_buf_write_lock(buf)
>> {
>>          for_each_vm_in_buf(buf, vm) {
>>                  ww_mutex_lock(vm->resv);
>>          }
>> }
>>
>> Ideally we'd track all these vms with something slightly less shoddy than
>> a linked list :-) Resizeable array is probably pretty good, I think we
>> only ever need to go from buf -> vm list, not the other way round. At
>> least in dma_resv/dma_buf code, driver code ofc needs to keep a list of
>> all bo bound to a vm somewhere. But that's probably a much bigger
>> datastructure for tracking vma offsets and mappings and other things on
>> top.
>>
>> Ofc to even just get there we'd need something like the sublock list to
>> keep track of all the additional locks we'd need for the writer lock. And
>> we'd need the release callback for backoff, so that we could also go
>> through the slowpath on a vm object that we're not holding a full
>> reference on. That also means vm need to be refcounted.
>>
>> And the list of vms on a buffer need to be protected with some lock and
>> the usual kref_get_unless_zero trickery.
>>
>> But with all this I think we can make the dma_buf_write_lock lock 100%
>> like the old per-buffer lock for everyone. And execbuf could switch over
>> to dma_buf_read_lock for shared buffers. Bonus points when the gpu context
>> just keeps track of a list of shared vm used by buffers in that context
>> ...
>>
>> That way we could make vm fastpath locking a la amdgpu opt-in, while
>> keeping everyone else on the per-object locking juices.
>>
>> Thoughts?

At least to me that sounds like a plan.

> One thing I just realized, which is nasty: The full (write) lock needs
> ww_acquire_ctx with this, because it needs to take a bunch of locks.
> Rolling that out everywhere is going to be nasty.

Why? Take a single write lock shouldn't be different to taking a single 
ww_mutex, or am I missing something?

> I guess though we could do a fallback and have a locally created
> ww_acquire_ctx if there's none passed in, with backoff entirely
> implemented within dma_resv_lock.

How should that work? As far as I understand it the ww_acquire_ctx must 
be kept existing until after the last of the locks it was used with is 
unlocked. Or do I see this incorrectly?

> -Daniel
>
>> Cheers, Daniel
>>
>> PS: Of course the write lock for these buffers is going to be terrible, so
>> every time you need to update fences for implicit sync on shared buffer
>> (well write fence at least) it's going to suck. We probably also want a
>> read_to_write_upgrade function, which also can be done easily with
>> ww_mutex magic.

I'm thinking that we probably sole want a read_to_write upgrade function.

Regards,
Christian.

>> --
>> Daniel Vetter
>> Software Engineer, Intel Corporation
>>
Daniel Vetter Feb. 27, 2020, 9:38 a.m. UTC | #22
On Thu, Feb 27, 2020 at 10:20 AM Christian König
<christian.koenig@amd.com> wrote:
> Am 26.02.20 um 17:32 schrieb Daniel Vetter:
> > On Tue, Feb 25, 2020 at 6:16 PM Daniel Vetter <daniel@ffwll.ch> wrote:
> >> On Mon, Feb 24, 2020 at 07:46:59PM +0100, Christian König wrote:
> >>> Am 23.02.20 um 17:54 schrieb Thomas Hellström (VMware):
> >>>> On 2/23/20 4:45 PM, Christian König wrote:
> >>>>> Am 21.02.20 um 18:12 schrieb Daniel Vetter:
> >>>>>> [SNIP]
> >>>>>> Yeah the Great Plan (tm) is to fully rely on ww_mutex slowly
> >>>>>> degenerating
> >>>>>> into essentially a global lock. But only when there's actual contention
> >>>>>> and thrashing.
> >>>>> Yes exactly. A really big problem in TTM is currently that we drop
> >>>>> the lock after evicting BOs because they tend to move in again
> >>>>> directly after that.
> >>>>>
> >>>>>  From practice I can also confirm that there is exactly zero benefit
> >>>>> from dropping locks early and reacquire them for example for the VM
> >>>>> page tables. That's just makes it more likely that somebody needs to
> >>>>> roll back and this is what we need to avoid in the first place.
> >>>> If you have a benchmarking setup available it would be very interesting
> >>>> for future reference to see how changing from WD to WW mutexes affects
> >>>> the roll back frequency. WW is known to cause rollbacks much less
> >>>> frequently but there is more work associated with each rollback.
> >>> Not of hand. To be honest I still have a hard time to get a grip on the
> >>> difference between WD and WW from the algorithm point of view. So I can't
> >>> judge that difference at all.
> >>>
> >>>>> Contention on BO locks during command submission is perfectly fine
> >>>>> as long as this is as lightweight as possible while we don't have
> >>>>> trashing. When we have trashing multi submission performance is best
> >>>>> archived to just favor a single process to finish its business and
> >>>>> block everybody else.
> >>>> Hmm. Sounds like we need a per-manager ww_rwsem protecting manager
> >>>> allocation, taken in write-mode then there's thrashing. In read-mode
> >>>> otherwise. That would limit the amount of "unnecessary" locks we'd have
> >>>> to keep and reduce unwanted side-effects, (see below):
> >>> Well per-manager (you mean per domain here don't you?) doesn't sound like
> >>> that useful because we rarely use only one domain, but I'm actually
> >>> questioning for quite a while if the per BO lock scheme was the right
> >>> approach.
> >>>
> >>> See from the performance aspect the closest to ideal solution I can think of
> >>> would be a ww_rwsem per user of a resource.
> >>>
> >>> In other words we don't lock BOs, but instead a list of all their users and
> >>> when you want to evict a BO you need to walk that list and inform all users
> >>> that the BO will be moving.
> >>>
> >>> During command submission you then have the fast path which rather just
> >>> grabs the read side of the user lock and check if all BOs are still in the
> >>> expected place.
> >>>
> >>> If some BOs were evicted you back off and start the slow path, e.g. maybe
> >>> even copy additional data from userspace then grab the write side of the
> >>> lock etc.. etc...
> >>>
> >>> That approach is similar to what we use in amdgpu with the per-VM BOs, but
> >>> goes a step further. Problem is that we are so used to per BO locks in the
> >>> kernel that this is probably not doable any more.
> >> Yeah I think it'd be nice to have the same approach for shared bo too. I
> >> guess what we could do is something like this (spinning your ww_rwmutex
> >> idea a bit further):
> >>
> >> dma_buf_read_lock(buf, vm)
> >> {
> >>          if (enabled(CONFIG_DEBUG_WW_MUTEX_SLOWPATH))
> >>          {
> >>                  check that vm is indeed listed in buf and splat if not
> >>          }
> >>
> >>          /* for a buf that's not shared in multiple vm we'd have buf->resv
> >>           * == vm->resv here */
> >>          return ww_mutex_lock(vm->resv);
> >> }
> >>
> >> dma_buf_write_lock(buf)
> >> {
> >>          for_each_vm_in_buf(buf, vm) {
> >>                  ww_mutex_lock(vm->resv);
> >>          }
> >> }
> >>
> >> Ideally we'd track all these vms with something slightly less shoddy than
> >> a linked list :-) Resizeable array is probably pretty good, I think we
> >> only ever need to go from buf -> vm list, not the other way round. At
> >> least in dma_resv/dma_buf code, driver code ofc needs to keep a list of
> >> all bo bound to a vm somewhere. But that's probably a much bigger
> >> datastructure for tracking vma offsets and mappings and other things on
> >> top.
> >>
> >> Ofc to even just get there we'd need something like the sublock list to
> >> keep track of all the additional locks we'd need for the writer lock. And
> >> we'd need the release callback for backoff, so that we could also go
> >> through the slowpath on a vm object that we're not holding a full
> >> reference on. That also means vm need to be refcounted.
> >>
> >> And the list of vms on a buffer need to be protected with some lock and
> >> the usual kref_get_unless_zero trickery.
> >>
> >> But with all this I think we can make the dma_buf_write_lock lock 100%
> >> like the old per-buffer lock for everyone. And execbuf could switch over
> >> to dma_buf_read_lock for shared buffers. Bonus points when the gpu context
> >> just keeps track of a list of shared vm used by buffers in that context
> >> ...
> >>
> >> That way we could make vm fastpath locking a la amdgpu opt-in, while
> >> keeping everyone else on the per-object locking juices.
> >>
> >> Thoughts?
>
> At least to me that sounds like a plan.
>
> > One thing I just realized, which is nasty: The full (write) lock needs
> > ww_acquire_ctx with this, because it needs to take a bunch of locks.
> > Rolling that out everywhere is going to be nasty.
>
> Why? Take a single write lock shouldn't be different to taking a single
> ww_mutex, or am I missing something?

There's no single write lock in my proposal. The write lock for a
buffer is "take all the vm locks this buffer is mapped into". Which
means we need to take multiple ww_mutex locks, which means backoff and
everything.

Otherwise the read lock of just taking the per vm dma_resv lock won't
work. And doing an true rw lock on each buffer feels a bit pointless,
since then execbuf is back to O(num_buffers). And that's the suck
we're trying to avoid.

> > I guess though we could do a fallback and have a locally created
> > ww_acquire_ctx if there's none passed in, with backoff entirely
> > implemented within dma_resv_lock.
>
> How should that work? As far as I understand it the ww_acquire_ctx must
> be kept existing until after the last of the locks it was used with is
> unlocked. Or do I see this incorrectly?

Ugh right.

I guess we could put the ww_acquire_ctx into the dma_resv so it
survives until the unlock. But that's a rather gross hack.
-Daniel

> > -Daniel
> >
> >> Cheers, Daniel
> >>
> >> PS: Of course the write lock for these buffers is going to be terrible, so
> >> every time you need to update fences for implicit sync on shared buffer
> >> (well write fence at least) it's going to suck. We probably also want a
> >> read_to_write_upgrade function, which also can be done easily with
> >> ww_mutex magic.
>
> I'm thinking that we probably sole want a read_to_write upgrade function.
>
> Regards,
> Christian.
>
> >> --
> >> Daniel Vetter
> >> Software Engineer, Intel Corporation
> >>
>

Patch
diff mbox series

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
index 770baba621b3..48de7624d49c 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
@@ -453,7 +453,71 @@  amdgpu_dma_buf_create_obj(struct drm_device *dev, struct dma_buf *dma_buf)
 	return ERR_PTR(ret);
 }
 
+/**
+ * amdgpu_dma_buf_move_notify - &attach.move_notify implementation
+ *
+ * @attach: the DMA-buf attachment
+ *
+ * Invalidate the DMA-buf attachment, making sure that the we re-create the
+ * mapping before the next use.
+ */
+static void
+amdgpu_dma_buf_move_notify(struct dma_buf_attachment *attach)
+{
+	struct drm_gem_object *obj = attach->importer_priv;
+	struct ww_acquire_ctx *ticket = dma_resv_locking_ctx(obj->resv);
+	struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
+	struct amdgpu_device *adev = amdgpu_ttm_adev(bo->tbo.bdev);
+	struct ttm_operation_ctx ctx = { false, false };
+	struct ttm_placement placement = {};
+	struct amdgpu_vm_bo_base *bo_base;
+	int r;
+
+	if (bo->tbo.mem.mem_type == TTM_PL_SYSTEM)
+		return;
+
+	r = ttm_bo_validate(&bo->tbo, &placement, &ctx);
+	if (r) {
+		DRM_ERROR("Failed to invalidate DMA-buf import (%d))\n", r);
+		return;
+	}
+
+	for (bo_base = bo->vm_bo; bo_base; bo_base = bo_base->next) {
+		struct amdgpu_vm *vm = bo_base->vm;
+		struct dma_resv *resv = vm->root.base.bo->tbo.base.resv;
+
+		if (ticket) {
+			/* When we get an error here it means that somebody
+			 * else is holding the VM lock and updating page tables
+			 * So we can just continue here.
+			 */
+			r = dma_resv_lock(resv, ticket);
+			if (r)
+				continue;
+
+		} else {
+			/* TODO: This is more problematic and we actually need
+			 * to allow page tables updates without holding the
+			 * lock.
+			 */
+			if (!dma_resv_trylock(resv))
+				continue;
+		}
+
+		r = amdgpu_vm_clear_freed(adev, vm, NULL);
+		if (!r)
+			r = amdgpu_vm_handle_moved(adev, vm);
+
+		if (r && r != -EBUSY)
+			DRM_ERROR("Failed to invalidate VM page tables (%d))\n",
+				  r);
+
+		dma_resv_unlock(resv);
+	}
+}
+
 static const struct dma_buf_attach_ops amdgpu_dma_buf_attach_ops = {
+	.move_notify = amdgpu_dma_buf_move_notify
 };
 
 /**
@@ -489,7 +553,7 @@  struct drm_gem_object *amdgpu_gem_prime_import(struct drm_device *dev,
 		return obj;
 
 	attach = dma_buf_dynamic_attach(dma_buf, dev->dev,
-					&amdgpu_dma_buf_attach_ops, NULL);
+					&amdgpu_dma_buf_attach_ops, obj);
 	if (IS_ERR(attach)) {
 		drm_gem_object_put(obj);
 		return ERR_CAST(attach);
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
index 8ae260822908..8c480c898b0d 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
@@ -926,6 +926,9 @@  int amdgpu_bo_pin_restricted(struct amdgpu_bo *bo, u32 domain,
 		return 0;
 	}
 
+	if (bo->tbo.base.import_attach)
+		dma_buf_pin(bo->tbo.base.import_attach);
+
 	bo->flags |= AMDGPU_GEM_CREATE_VRAM_CONTIGUOUS;
 	/* force to pin into visible video ram */
 	if (!(bo->flags & AMDGPU_GEM_CREATE_NO_CPU_ACCESS))
@@ -1009,6 +1012,9 @@  int amdgpu_bo_unpin(struct amdgpu_bo *bo)
 
 	amdgpu_bo_subtract_pin_size(bo);
 
+	if (bo->tbo.base.import_attach)
+		dma_buf_unpin(bo->tbo.base.import_attach);
+
 	for (i = 0; i < bo->placement.num_placement; i++) {
 		bo->placements[i].lpfn = 0;
 		bo->placements[i].flags &= ~TTM_PL_FLAG_NO_EVICT;