[RFC,03/13] vhost: batching fetches
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Message ID 20200602130543.578420-4-mst@redhat.com
State New
Headers show
Series
  • vhost: format independence
Related show

Commit Message

Michael S. Tsirkin June 2, 2020, 1:06 p.m. UTC
With this patch applied, new and old code perform identically.

Lots of extra optimizations are now possible, e.g.
we can fetch multiple heads with copy_from/to_user now.
We can get rid of maintaining the log array.  Etc etc.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Link: https://lore.kernel.org/r/20200401183118.8334-4-eperezma@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 drivers/vhost/test.c  |  2 +-
 drivers/vhost/vhost.c | 47 ++++++++++++++++++++++++++++++++++++++-----
 drivers/vhost/vhost.h |  5 ++++-
 3 files changed, 47 insertions(+), 7 deletions(-)

Comments

Jason Wang June 3, 2020, 7:27 a.m. UTC | #1
On 2020/6/2 下午9:06, Michael S. Tsirkin wrote:
> With this patch applied, new and old code perform identically.
>
> Lots of extra optimizations are now possible, e.g.
> we can fetch multiple heads with copy_from/to_user now.
> We can get rid of maintaining the log array.  Etc etc.
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> Link: https://lore.kernel.org/r/20200401183118.8334-4-eperezma@redhat.com
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>   drivers/vhost/test.c  |  2 +-
>   drivers/vhost/vhost.c | 47 ++++++++++++++++++++++++++++++++++++++-----
>   drivers/vhost/vhost.h |  5 ++++-
>   3 files changed, 47 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
> index 9a3a09005e03..02806d6f84ef 100644
> --- a/drivers/vhost/test.c
> +++ b/drivers/vhost/test.c
> @@ -119,7 +119,7 @@ static int vhost_test_open(struct inode *inode, struct file *f)
>   	dev = &n->dev;
>   	vqs[VHOST_TEST_VQ] = &n->vqs[VHOST_TEST_VQ];
>   	n->vqs[VHOST_TEST_VQ].handle_kick = handle_vq_kick;
> -	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV,
> +	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV + 64,
>   		       VHOST_TEST_PKT_WEIGHT, VHOST_TEST_WEIGHT, NULL);
>   
>   	f->private_data = n;
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 8f9a07282625..aca2a5b0d078 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -299,6 +299,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>   {
>   	vq->num = 1;
>   	vq->ndescs = 0;
> +	vq->first_desc = 0;
>   	vq->desc = NULL;
>   	vq->avail = NULL;
>   	vq->used = NULL;
> @@ -367,6 +368,11 @@ static int vhost_worker(void *data)
>   	return 0;
>   }
>   
> +static int vhost_vq_num_batch_descs(struct vhost_virtqueue *vq)
> +{
> +	return vq->max_descs - UIO_MAXIOV;
> +}


1 descriptor does not mean 1 iov, e.g userspace may pass several 1 byte 
length memory regions for us to translate.


> +
>   static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq)
>   {
>   	kfree(vq->descs);
> @@ -389,6 +395,9 @@ static long vhost_dev_alloc_iovecs(struct vhost_dev *dev)
>   	for (i = 0; i < dev->nvqs; ++i) {
>   		vq = dev->vqs[i];
>   		vq->max_descs = dev->iov_limit;
> +		if (vhost_vq_num_batch_descs(vq) < 0) {
> +			return -EINVAL;
> +		}
>   		vq->descs = kmalloc_array(vq->max_descs,
>   					  sizeof(*vq->descs),
>   					  GFP_KERNEL);
> @@ -1570,6 +1579,7 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
>   		vq->last_avail_idx = s.num;
>   		/* Forget the cached index value. */
>   		vq->avail_idx = vq->last_avail_idx;
> +		vq->ndescs = vq->first_desc = 0;
>   		break;
>   	case VHOST_GET_VRING_BASE:
>   		s.index = idx;
> @@ -2136,7 +2146,7 @@ static int fetch_indirect_descs(struct vhost_virtqueue *vq,
>   	return 0;
>   }
>   
> -static int fetch_descs(struct vhost_virtqueue *vq)
> +static int fetch_buf(struct vhost_virtqueue *vq)
>   {
>   	unsigned int i, head, found = 0;
>   	struct vhost_desc *last;
> @@ -2149,7 +2159,11 @@ static int fetch_descs(struct vhost_virtqueue *vq)
>   	/* Check it isn't doing very strange things with descriptor numbers. */
>   	last_avail_idx = vq->last_avail_idx;
>   
> -	if (vq->avail_idx == vq->last_avail_idx) {
> +	if (unlikely(vq->avail_idx == vq->last_avail_idx)) {
> +		/* If we already have work to do, don't bother re-checking. */
> +		if (likely(vq->ndescs))
> +			return vq->num;
> +
>   		if (unlikely(vhost_get_avail_idx(vq, &avail_idx))) {
>   			vq_err(vq, "Failed to access avail idx at %p\n",
>   				&vq->avail->idx);
> @@ -2240,6 +2254,24 @@ static int fetch_descs(struct vhost_virtqueue *vq)
>   	return 0;
>   }
>   
> +static int fetch_descs(struct vhost_virtqueue *vq)
> +{
> +	int ret = 0;
> +
> +	if (unlikely(vq->first_desc >= vq->ndescs)) {
> +		vq->first_desc = 0;
> +		vq->ndescs = 0;
> +	}
> +
> +	if (vq->ndescs)
> +		return 0;
> +
> +	while (!ret && vq->ndescs <= vhost_vq_num_batch_descs(vq))
> +		ret = fetch_buf(vq);
> +
> +	return vq->ndescs ? 0 : ret;
> +}
> +
>   /* This looks in the virtqueue and for the first available buffer, and converts
>    * it to an iovec for convenient access.  Since descriptors consist of some
>    * number of output then some number of input descriptors, it's actually two
> @@ -2265,7 +2297,7 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
>   	if (unlikely(log))
>   		*log_num = 0;
>   
> -	for (i = 0; i < vq->ndescs; ++i) {
> +	for (i = vq->first_desc; i < vq->ndescs; ++i) {
>   		unsigned iov_count = *in_num + *out_num;
>   		struct vhost_desc *desc = &vq->descs[i];
>   		int access;
> @@ -2311,14 +2343,19 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
>   		}
>   
>   		ret = desc->id;
> +
> +		if (!(desc->flags & VRING_DESC_F_NEXT))
> +			break;
>   	}
>   
> -	vq->ndescs = 0;
> +	vq->first_desc = i + 1;
>   
>   	return ret;
>   
>   err:
> -	vhost_discard_vq_desc(vq, 1);
> +	for (i = vq->first_desc; i < vq->ndescs; ++i)
> +		if (!(vq->descs[i].flags & VRING_DESC_F_NEXT))
> +			vhost_discard_vq_desc(vq, 1);
>   	vq->ndescs = 0;
>   
>   	return ret;
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 76356edee8e5..a67bda9792ec 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -81,6 +81,7 @@ struct vhost_virtqueue {
>   
>   	struct vhost_desc *descs;
>   	int ndescs;
> +	int first_desc;
>   	int max_descs;
>   
>   	struct file *kick;
> @@ -229,7 +230,7 @@ void vhost_iotlb_map_free(struct vhost_iotlb *iotlb,
>   			  struct vhost_iotlb_map *map);
>   
>   #define vq_err(vq, fmt, ...) do {                                  \
> -		pr_debug(pr_fmt(fmt), ##__VA_ARGS__);       \
> +		pr_err(pr_fmt(fmt), ##__VA_ARGS__);       \


Need a separate patch for this?

Thanks


>   		if ((vq)->error_ctx)                               \
>   				eventfd_signal((vq)->error_ctx, 1);\
>   	} while (0)
> @@ -255,6 +256,8 @@ static inline void vhost_vq_set_backend(struct vhost_virtqueue *vq,
>   					void *private_data)
>   {
>   	vq->private_data = private_data;
> +	vq->ndescs = 0;
> +	vq->first_desc = 0;
>   }
>   
>   /**
Michael S. Tsirkin June 4, 2020, 8:59 a.m. UTC | #2
On Wed, Jun 03, 2020 at 03:27:39PM +0800, Jason Wang wrote:
> 
> On 2020/6/2 下午9:06, Michael S. Tsirkin wrote:
> > With this patch applied, new and old code perform identically.
> > 
> > Lots of extra optimizations are now possible, e.g.
> > we can fetch multiple heads with copy_from/to_user now.
> > We can get rid of maintaining the log array.  Etc etc.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > Link: https://lore.kernel.org/r/20200401183118.8334-4-eperezma@redhat.com
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >   drivers/vhost/test.c  |  2 +-
> >   drivers/vhost/vhost.c | 47 ++++++++++++++++++++++++++++++++++++++-----
> >   drivers/vhost/vhost.h |  5 ++++-
> >   3 files changed, 47 insertions(+), 7 deletions(-)
> > 
> > diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
> > index 9a3a09005e03..02806d6f84ef 100644
> > --- a/drivers/vhost/test.c
> > +++ b/drivers/vhost/test.c
> > @@ -119,7 +119,7 @@ static int vhost_test_open(struct inode *inode, struct file *f)
> >   	dev = &n->dev;
> >   	vqs[VHOST_TEST_VQ] = &n->vqs[VHOST_TEST_VQ];
> >   	n->vqs[VHOST_TEST_VQ].handle_kick = handle_vq_kick;
> > -	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV,
> > +	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV + 64,
> >   		       VHOST_TEST_PKT_WEIGHT, VHOST_TEST_WEIGHT, NULL);
> >   	f->private_data = n;
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 8f9a07282625..aca2a5b0d078 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -299,6 +299,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
> >   {
> >   	vq->num = 1;
> >   	vq->ndescs = 0;
> > +	vq->first_desc = 0;
> >   	vq->desc = NULL;
> >   	vq->avail = NULL;
> >   	vq->used = NULL;
> > @@ -367,6 +368,11 @@ static int vhost_worker(void *data)
> >   	return 0;
> >   }
> > +static int vhost_vq_num_batch_descs(struct vhost_virtqueue *vq)
> > +{
> > +	return vq->max_descs - UIO_MAXIOV;
> > +}
> 
> 
> 1 descriptor does not mean 1 iov, e.g userspace may pass several 1 byte
> length memory regions for us to translate.
> 


Yes but I don't see the relevance. This tells us how many descriptors to
batch, not how many IOVs.

> > +
> >   static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq)
> >   {
> >   	kfree(vq->descs);
> > @@ -389,6 +395,9 @@ static long vhost_dev_alloc_iovecs(struct vhost_dev *dev)
> >   	for (i = 0; i < dev->nvqs; ++i) {
> >   		vq = dev->vqs[i];
> >   		vq->max_descs = dev->iov_limit;
> > +		if (vhost_vq_num_batch_descs(vq) < 0) {
> > +			return -EINVAL;
> > +		}
> >   		vq->descs = kmalloc_array(vq->max_descs,
> >   					  sizeof(*vq->descs),
> >   					  GFP_KERNEL);
> > @@ -1570,6 +1579,7 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
> >   		vq->last_avail_idx = s.num;
> >   		/* Forget the cached index value. */
> >   		vq->avail_idx = vq->last_avail_idx;
> > +		vq->ndescs = vq->first_desc = 0;
> >   		break;
> >   	case VHOST_GET_VRING_BASE:
> >   		s.index = idx;
> > @@ -2136,7 +2146,7 @@ static int fetch_indirect_descs(struct vhost_virtqueue *vq,
> >   	return 0;
> >   }
> > -static int fetch_descs(struct vhost_virtqueue *vq)
> > +static int fetch_buf(struct vhost_virtqueue *vq)
> >   {
> >   	unsigned int i, head, found = 0;
> >   	struct vhost_desc *last;
> > @@ -2149,7 +2159,11 @@ static int fetch_descs(struct vhost_virtqueue *vq)
> >   	/* Check it isn't doing very strange things with descriptor numbers. */
> >   	last_avail_idx = vq->last_avail_idx;
> > -	if (vq->avail_idx == vq->last_avail_idx) {
> > +	if (unlikely(vq->avail_idx == vq->last_avail_idx)) {
> > +		/* If we already have work to do, don't bother re-checking. */
> > +		if (likely(vq->ndescs))
> > +			return vq->num;
> > +
> >   		if (unlikely(vhost_get_avail_idx(vq, &avail_idx))) {
> >   			vq_err(vq, "Failed to access avail idx at %p\n",
> >   				&vq->avail->idx);
> > @@ -2240,6 +2254,24 @@ static int fetch_descs(struct vhost_virtqueue *vq)
> >   	return 0;
> >   }
> > +static int fetch_descs(struct vhost_virtqueue *vq)
> > +{
> > +	int ret = 0;
> > +
> > +	if (unlikely(vq->first_desc >= vq->ndescs)) {
> > +		vq->first_desc = 0;
> > +		vq->ndescs = 0;
> > +	}
> > +
> > +	if (vq->ndescs)
> > +		return 0;
> > +
> > +	while (!ret && vq->ndescs <= vhost_vq_num_batch_descs(vq))
> > +		ret = fetch_buf(vq);
> > +
> > +	return vq->ndescs ? 0 : ret;
> > +}
> > +
> >   /* This looks in the virtqueue and for the first available buffer, and converts
> >    * it to an iovec for convenient access.  Since descriptors consist of some
> >    * number of output then some number of input descriptors, it's actually two
> > @@ -2265,7 +2297,7 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> >   	if (unlikely(log))
> >   		*log_num = 0;
> > -	for (i = 0; i < vq->ndescs; ++i) {
> > +	for (i = vq->first_desc; i < vq->ndescs; ++i) {
> >   		unsigned iov_count = *in_num + *out_num;
> >   		struct vhost_desc *desc = &vq->descs[i];
> >   		int access;
> > @@ -2311,14 +2343,19 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> >   		}
> >   		ret = desc->id;
> > +
> > +		if (!(desc->flags & VRING_DESC_F_NEXT))
> > +			break;
> >   	}
> > -	vq->ndescs = 0;
> > +	vq->first_desc = i + 1;
> >   	return ret;
> >   err:
> > -	vhost_discard_vq_desc(vq, 1);
> > +	for (i = vq->first_desc; i < vq->ndescs; ++i)
> > +		if (!(vq->descs[i].flags & VRING_DESC_F_NEXT))
> > +			vhost_discard_vq_desc(vq, 1);
> >   	vq->ndescs = 0;
> >   	return ret;
> > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > index 76356edee8e5..a67bda9792ec 100644
> > --- a/drivers/vhost/vhost.h
> > +++ b/drivers/vhost/vhost.h
> > @@ -81,6 +81,7 @@ struct vhost_virtqueue {
> >   	struct vhost_desc *descs;
> >   	int ndescs;
> > +	int first_desc;
> >   	int max_descs;
> >   	struct file *kick;
> > @@ -229,7 +230,7 @@ void vhost_iotlb_map_free(struct vhost_iotlb *iotlb,
> >   			  struct vhost_iotlb_map *map);
> >   #define vq_err(vq, fmt, ...) do {                                  \
> > -		pr_debug(pr_fmt(fmt), ##__VA_ARGS__);       \
> > +		pr_err(pr_fmt(fmt), ##__VA_ARGS__);       \
> 
> 
> Need a separate patch for this?
> 
> Thanks


Oh that's a debugging thing. I will drop it.

> 
> >   		if ((vq)->error_ctx)                               \
> >   				eventfd_signal((vq)->error_ctx, 1);\
> >   	} while (0)
> > @@ -255,6 +256,8 @@ static inline void vhost_vq_set_backend(struct vhost_virtqueue *vq,
> >   					void *private_data)
> >   {
> >   	vq->private_data = private_data;
> > +	vq->ndescs = 0;
> > +	vq->first_desc = 0;
> >   }
> >   /**
Jason Wang June 5, 2020, 3:40 a.m. UTC | #3
On 2020/6/4 下午4:59, Michael S. Tsirkin wrote:
> On Wed, Jun 03, 2020 at 03:27:39PM +0800, Jason Wang wrote:
>> On 2020/6/2 下午9:06, Michael S. Tsirkin wrote:
>>> With this patch applied, new and old code perform identically.
>>>
>>> Lots of extra optimizations are now possible, e.g.
>>> we can fetch multiple heads with copy_from/to_user now.
>>> We can get rid of maintaining the log array.  Etc etc.
>>>
>>> Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
>>> Signed-off-by: Eugenio Pérez<eperezma@redhat.com>
>>> Link:https://lore.kernel.org/r/20200401183118.8334-4-eperezma@redhat.com
>>> Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
>>> ---
>>>    drivers/vhost/test.c  |  2 +-
>>>    drivers/vhost/vhost.c | 47 ++++++++++++++++++++++++++++++++++++++-----
>>>    drivers/vhost/vhost.h |  5 ++++-
>>>    3 files changed, 47 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
>>> index 9a3a09005e03..02806d6f84ef 100644
>>> --- a/drivers/vhost/test.c
>>> +++ b/drivers/vhost/test.c
>>> @@ -119,7 +119,7 @@ static int vhost_test_open(struct inode *inode, struct file *f)
>>>    	dev = &n->dev;
>>>    	vqs[VHOST_TEST_VQ] = &n->vqs[VHOST_TEST_VQ];
>>>    	n->vqs[VHOST_TEST_VQ].handle_kick = handle_vq_kick;
>>> -	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV,
>>> +	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV + 64,
>>>    		       VHOST_TEST_PKT_WEIGHT, VHOST_TEST_WEIGHT, NULL);
>>>    	f->private_data = n;
>>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>>> index 8f9a07282625..aca2a5b0d078 100644
>>> --- a/drivers/vhost/vhost.c
>>> +++ b/drivers/vhost/vhost.c
>>> @@ -299,6 +299,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>>>    {
>>>    	vq->num = 1;
>>>    	vq->ndescs = 0;
>>> +	vq->first_desc = 0;
>>>    	vq->desc = NULL;
>>>    	vq->avail = NULL;
>>>    	vq->used = NULL;
>>> @@ -367,6 +368,11 @@ static int vhost_worker(void *data)
>>>    	return 0;
>>>    }
>>> +static int vhost_vq_num_batch_descs(struct vhost_virtqueue *vq)
>>> +{
>>> +	return vq->max_descs - UIO_MAXIOV;
>>> +}
>> 1 descriptor does not mean 1 iov, e.g userspace may pass several 1 byte
>> length memory regions for us to translate.
>>
> Yes but I don't see the relevance. This tells us how many descriptors to
> batch, not how many IOVs.


Yes, but questions are:

- this introduce another obstacle to support more than 1K queue size
- if we support 1K queue size, does it mean we need to cache 1K 
descriptors, which seems a large stress on the cache

Thanks


>
Michael S. Tsirkin June 7, 2020, 1:57 p.m. UTC | #4
On Fri, Jun 05, 2020 at 11:40:17AM +0800, Jason Wang wrote:
> 
> On 2020/6/4 下午4:59, Michael S. Tsirkin wrote:
> > On Wed, Jun 03, 2020 at 03:27:39PM +0800, Jason Wang wrote:
> > > On 2020/6/2 下午9:06, Michael S. Tsirkin wrote:
> > > > With this patch applied, new and old code perform identically.
> > > > 
> > > > Lots of extra optimizations are now possible, e.g.
> > > > we can fetch multiple heads with copy_from/to_user now.
> > > > We can get rid of maintaining the log array.  Etc etc.
> > > > 
> > > > Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
> > > > Signed-off-by: Eugenio Pérez<eperezma@redhat.com>
> > > > Link:https://lore.kernel.org/r/20200401183118.8334-4-eperezma@redhat.com
> > > > Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
> > > > ---
> > > >    drivers/vhost/test.c  |  2 +-
> > > >    drivers/vhost/vhost.c | 47 ++++++++++++++++++++++++++++++++++++++-----
> > > >    drivers/vhost/vhost.h |  5 ++++-
> > > >    3 files changed, 47 insertions(+), 7 deletions(-)
> > > > 
> > > > diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
> > > > index 9a3a09005e03..02806d6f84ef 100644
> > > > --- a/drivers/vhost/test.c
> > > > +++ b/drivers/vhost/test.c
> > > > @@ -119,7 +119,7 @@ static int vhost_test_open(struct inode *inode, struct file *f)
> > > >    	dev = &n->dev;
> > > >    	vqs[VHOST_TEST_VQ] = &n->vqs[VHOST_TEST_VQ];
> > > >    	n->vqs[VHOST_TEST_VQ].handle_kick = handle_vq_kick;
> > > > -	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV,
> > > > +	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV + 64,
> > > >    		       VHOST_TEST_PKT_WEIGHT, VHOST_TEST_WEIGHT, NULL);
> > > >    	f->private_data = n;
> > > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > > index 8f9a07282625..aca2a5b0d078 100644
> > > > --- a/drivers/vhost/vhost.c
> > > > +++ b/drivers/vhost/vhost.c
> > > > @@ -299,6 +299,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
> > > >    {
> > > >    	vq->num = 1;
> > > >    	vq->ndescs = 0;
> > > > +	vq->first_desc = 0;
> > > >    	vq->desc = NULL;
> > > >    	vq->avail = NULL;
> > > >    	vq->used = NULL;
> > > > @@ -367,6 +368,11 @@ static int vhost_worker(void *data)
> > > >    	return 0;
> > > >    }
> > > > +static int vhost_vq_num_batch_descs(struct vhost_virtqueue *vq)
> > > > +{
> > > > +	return vq->max_descs - UIO_MAXIOV;
> > > > +}
> > > 1 descriptor does not mean 1 iov, e.g userspace may pass several 1 byte
> > > length memory regions for us to translate.
> > > 
> > Yes but I don't see the relevance. This tells us how many descriptors to
> > batch, not how many IOVs.
> 
> 
> Yes, but questions are:
> 
> - this introduce another obstacle to support more than 1K queue size
> - if we support 1K queue size, does it mean we need to cache 1K descriptors,
> which seems a large stress on the cache
> 
> Thanks
> 
> 
> > 

Still don't understand the relevance. We support up to 1K descriptors
per buffer just for IOV since we always did. This adds 64 more
descriptors - is that a big deal?
Jason Wang June 8, 2020, 3:35 a.m. UTC | #5
On 2020/6/7 下午9:57, Michael S. Tsirkin wrote:
> On Fri, Jun 05, 2020 at 11:40:17AM +0800, Jason Wang wrote:
>> On 2020/6/4 下午4:59, Michael S. Tsirkin wrote:
>>> On Wed, Jun 03, 2020 at 03:27:39PM +0800, Jason Wang wrote:
>>>> On 2020/6/2 下午9:06, Michael S. Tsirkin wrote:
>>>>> With this patch applied, new and old code perform identically.
>>>>>
>>>>> Lots of extra optimizations are now possible, e.g.
>>>>> we can fetch multiple heads with copy_from/to_user now.
>>>>> We can get rid of maintaining the log array.  Etc etc.
>>>>>
>>>>> Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
>>>>> Signed-off-by: Eugenio Pérez<eperezma@redhat.com>
>>>>> Link:https://lore.kernel.org/r/20200401183118.8334-4-eperezma@redhat.com
>>>>> Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
>>>>> ---
>>>>>     drivers/vhost/test.c  |  2 +-
>>>>>     drivers/vhost/vhost.c | 47 ++++++++++++++++++++++++++++++++++++++-----
>>>>>     drivers/vhost/vhost.h |  5 ++++-
>>>>>     3 files changed, 47 insertions(+), 7 deletions(-)
>>>>>
>>>>> diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
>>>>> index 9a3a09005e03..02806d6f84ef 100644
>>>>> --- a/drivers/vhost/test.c
>>>>> +++ b/drivers/vhost/test.c
>>>>> @@ -119,7 +119,7 @@ static int vhost_test_open(struct inode *inode, struct file *f)
>>>>>     	dev = &n->dev;
>>>>>     	vqs[VHOST_TEST_VQ] = &n->vqs[VHOST_TEST_VQ];
>>>>>     	n->vqs[VHOST_TEST_VQ].handle_kick = handle_vq_kick;
>>>>> -	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV,
>>>>> +	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV + 64,
>>>>>     		       VHOST_TEST_PKT_WEIGHT, VHOST_TEST_WEIGHT, NULL);
>>>>>     	f->private_data = n;
>>>>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>>>>> index 8f9a07282625..aca2a5b0d078 100644
>>>>> --- a/drivers/vhost/vhost.c
>>>>> +++ b/drivers/vhost/vhost.c
>>>>> @@ -299,6 +299,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>>>>>     {
>>>>>     	vq->num = 1;
>>>>>     	vq->ndescs = 0;
>>>>> +	vq->first_desc = 0;
>>>>>     	vq->desc = NULL;
>>>>>     	vq->avail = NULL;
>>>>>     	vq->used = NULL;
>>>>> @@ -367,6 +368,11 @@ static int vhost_worker(void *data)
>>>>>     	return 0;
>>>>>     }
>>>>> +static int vhost_vq_num_batch_descs(struct vhost_virtqueue *vq)
>>>>> +{
>>>>> +	return vq->max_descs - UIO_MAXIOV;
>>>>> +}
>>>> 1 descriptor does not mean 1 iov, e.g userspace may pass several 1 byte
>>>> length memory regions for us to translate.
>>>>
>>> Yes but I don't see the relevance. This tells us how many descriptors to
>>> batch, not how many IOVs.
>> Yes, but questions are:
>>
>> - this introduce another obstacle to support more than 1K queue size
>> - if we support 1K queue size, does it mean we need to cache 1K descriptors,
>> which seems a large stress on the cache
>>
>> Thanks
>>
>>
> Still don't understand the relevance. We support up to 1K descriptors
> per buffer just for IOV since we always did. This adds 64 more
> descriptors - is that a big deal?


If I understanding correctly, for net, the code tries to batch 
descriptors for at last one packet.

If we allow 1K queue size then we allow a packet that consists of 1K 
descriptors. Then we need to cache 1K descriptors.

Thanks
Michael S. Tsirkin June 8, 2020, 6:01 a.m. UTC | #6
On Mon, Jun 08, 2020 at 11:35:40AM +0800, Jason Wang wrote:
> 
> On 2020/6/7 下午9:57, Michael S. Tsirkin wrote:
> > On Fri, Jun 05, 2020 at 11:40:17AM +0800, Jason Wang wrote:
> > > On 2020/6/4 下午4:59, Michael S. Tsirkin wrote:
> > > > On Wed, Jun 03, 2020 at 03:27:39PM +0800, Jason Wang wrote:
> > > > > On 2020/6/2 下午9:06, Michael S. Tsirkin wrote:
> > > > > > With this patch applied, new and old code perform identically.
> > > > > > 
> > > > > > Lots of extra optimizations are now possible, e.g.
> > > > > > we can fetch multiple heads with copy_from/to_user now.
> > > > > > We can get rid of maintaining the log array.  Etc etc.
> > > > > > 
> > > > > > Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
> > > > > > Signed-off-by: Eugenio Pérez<eperezma@redhat.com>
> > > > > > Link:https://lore.kernel.org/r/20200401183118.8334-4-eperezma@redhat.com
> > > > > > Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
> > > > > > ---
> > > > > >     drivers/vhost/test.c  |  2 +-
> > > > > >     drivers/vhost/vhost.c | 47 ++++++++++++++++++++++++++++++++++++++-----
> > > > > >     drivers/vhost/vhost.h |  5 ++++-
> > > > > >     3 files changed, 47 insertions(+), 7 deletions(-)
> > > > > > 
> > > > > > diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
> > > > > > index 9a3a09005e03..02806d6f84ef 100644
> > > > > > --- a/drivers/vhost/test.c
> > > > > > +++ b/drivers/vhost/test.c
> > > > > > @@ -119,7 +119,7 @@ static int vhost_test_open(struct inode *inode, struct file *f)
> > > > > >     	dev = &n->dev;
> > > > > >     	vqs[VHOST_TEST_VQ] = &n->vqs[VHOST_TEST_VQ];
> > > > > >     	n->vqs[VHOST_TEST_VQ].handle_kick = handle_vq_kick;
> > > > > > -	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV,
> > > > > > +	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV + 64,
> > > > > >     		       VHOST_TEST_PKT_WEIGHT, VHOST_TEST_WEIGHT, NULL);
> > > > > >     	f->private_data = n;
> > > > > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > > > > index 8f9a07282625..aca2a5b0d078 100644
> > > > > > --- a/drivers/vhost/vhost.c
> > > > > > +++ b/drivers/vhost/vhost.c
> > > > > > @@ -299,6 +299,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
> > > > > >     {
> > > > > >     	vq->num = 1;
> > > > > >     	vq->ndescs = 0;
> > > > > > +	vq->first_desc = 0;
> > > > > >     	vq->desc = NULL;
> > > > > >     	vq->avail = NULL;
> > > > > >     	vq->used = NULL;
> > > > > > @@ -367,6 +368,11 @@ static int vhost_worker(void *data)
> > > > > >     	return 0;
> > > > > >     }
> > > > > > +static int vhost_vq_num_batch_descs(struct vhost_virtqueue *vq)
> > > > > > +{
> > > > > > +	return vq->max_descs - UIO_MAXIOV;
> > > > > > +}
> > > > > 1 descriptor does not mean 1 iov, e.g userspace may pass several 1 byte
> > > > > length memory regions for us to translate.
> > > > > 
> > > > Yes but I don't see the relevance. This tells us how many descriptors to
> > > > batch, not how many IOVs.
> > > Yes, but questions are:
> > > 
> > > - this introduce another obstacle to support more than 1K queue size
> > > - if we support 1K queue size, does it mean we need to cache 1K descriptors,
> > > which seems a large stress on the cache
> > > 
> > > Thanks
> > > 
> > > 
> > Still don't understand the relevance. We support up to 1K descriptors
> > per buffer just for IOV since we always did. This adds 64 more
> > descriptors - is that a big deal?
> 
> 
> If I understanding correctly, for net, the code tries to batch descriptors
> for at last one packet.
> 
> If we allow 1K queue size then we allow a packet that consists of 1K
> descriptors. Then we need to cache 1K descriptors.
> 
> Thanks

That case is already so pathological, I am not at all worried about
it performing well.

Patch
diff mbox series

diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
index 9a3a09005e03..02806d6f84ef 100644
--- a/drivers/vhost/test.c
+++ b/drivers/vhost/test.c
@@ -119,7 +119,7 @@  static int vhost_test_open(struct inode *inode, struct file *f)
 	dev = &n->dev;
 	vqs[VHOST_TEST_VQ] = &n->vqs[VHOST_TEST_VQ];
 	n->vqs[VHOST_TEST_VQ].handle_kick = handle_vq_kick;
-	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV,
+	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV + 64,
 		       VHOST_TEST_PKT_WEIGHT, VHOST_TEST_WEIGHT, NULL);
 
 	f->private_data = n;
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 8f9a07282625..aca2a5b0d078 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -299,6 +299,7 @@  static void vhost_vq_reset(struct vhost_dev *dev,
 {
 	vq->num = 1;
 	vq->ndescs = 0;
+	vq->first_desc = 0;
 	vq->desc = NULL;
 	vq->avail = NULL;
 	vq->used = NULL;
@@ -367,6 +368,11 @@  static int vhost_worker(void *data)
 	return 0;
 }
 
+static int vhost_vq_num_batch_descs(struct vhost_virtqueue *vq)
+{
+	return vq->max_descs - UIO_MAXIOV;
+}
+
 static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq)
 {
 	kfree(vq->descs);
@@ -389,6 +395,9 @@  static long vhost_dev_alloc_iovecs(struct vhost_dev *dev)
 	for (i = 0; i < dev->nvqs; ++i) {
 		vq = dev->vqs[i];
 		vq->max_descs = dev->iov_limit;
+		if (vhost_vq_num_batch_descs(vq) < 0) {
+			return -EINVAL;
+		}
 		vq->descs = kmalloc_array(vq->max_descs,
 					  sizeof(*vq->descs),
 					  GFP_KERNEL);
@@ -1570,6 +1579,7 @@  long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
 		vq->last_avail_idx = s.num;
 		/* Forget the cached index value. */
 		vq->avail_idx = vq->last_avail_idx;
+		vq->ndescs = vq->first_desc = 0;
 		break;
 	case VHOST_GET_VRING_BASE:
 		s.index = idx;
@@ -2136,7 +2146,7 @@  static int fetch_indirect_descs(struct vhost_virtqueue *vq,
 	return 0;
 }
 
-static int fetch_descs(struct vhost_virtqueue *vq)
+static int fetch_buf(struct vhost_virtqueue *vq)
 {
 	unsigned int i, head, found = 0;
 	struct vhost_desc *last;
@@ -2149,7 +2159,11 @@  static int fetch_descs(struct vhost_virtqueue *vq)
 	/* Check it isn't doing very strange things with descriptor numbers. */
 	last_avail_idx = vq->last_avail_idx;
 
-	if (vq->avail_idx == vq->last_avail_idx) {
+	if (unlikely(vq->avail_idx == vq->last_avail_idx)) {
+		/* If we already have work to do, don't bother re-checking. */
+		if (likely(vq->ndescs))
+			return vq->num;
+
 		if (unlikely(vhost_get_avail_idx(vq, &avail_idx))) {
 			vq_err(vq, "Failed to access avail idx at %p\n",
 				&vq->avail->idx);
@@ -2240,6 +2254,24 @@  static int fetch_descs(struct vhost_virtqueue *vq)
 	return 0;
 }
 
+static int fetch_descs(struct vhost_virtqueue *vq)
+{
+	int ret = 0;
+
+	if (unlikely(vq->first_desc >= vq->ndescs)) {
+		vq->first_desc = 0;
+		vq->ndescs = 0;
+	}
+
+	if (vq->ndescs)
+		return 0;
+
+	while (!ret && vq->ndescs <= vhost_vq_num_batch_descs(vq))
+		ret = fetch_buf(vq);
+
+	return vq->ndescs ? 0 : ret;
+}
+
 /* This looks in the virtqueue and for the first available buffer, and converts
  * it to an iovec for convenient access.  Since descriptors consist of some
  * number of output then some number of input descriptors, it's actually two
@@ -2265,7 +2297,7 @@  int vhost_get_vq_desc(struct vhost_virtqueue *vq,
 	if (unlikely(log))
 		*log_num = 0;
 
-	for (i = 0; i < vq->ndescs; ++i) {
+	for (i = vq->first_desc; i < vq->ndescs; ++i) {
 		unsigned iov_count = *in_num + *out_num;
 		struct vhost_desc *desc = &vq->descs[i];
 		int access;
@@ -2311,14 +2343,19 @@  int vhost_get_vq_desc(struct vhost_virtqueue *vq,
 		}
 
 		ret = desc->id;
+
+		if (!(desc->flags & VRING_DESC_F_NEXT))
+			break;
 	}
 
-	vq->ndescs = 0;
+	vq->first_desc = i + 1;
 
 	return ret;
 
 err:
-	vhost_discard_vq_desc(vq, 1);
+	for (i = vq->first_desc; i < vq->ndescs; ++i)
+		if (!(vq->descs[i].flags & VRING_DESC_F_NEXT))
+			vhost_discard_vq_desc(vq, 1);
 	vq->ndescs = 0;
 
 	return ret;
diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index 76356edee8e5..a67bda9792ec 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -81,6 +81,7 @@  struct vhost_virtqueue {
 
 	struct vhost_desc *descs;
 	int ndescs;
+	int first_desc;
 	int max_descs;
 
 	struct file *kick;
@@ -229,7 +230,7 @@  void vhost_iotlb_map_free(struct vhost_iotlb *iotlb,
 			  struct vhost_iotlb_map *map);
 
 #define vq_err(vq, fmt, ...) do {                                  \
-		pr_debug(pr_fmt(fmt), ##__VA_ARGS__);       \
+		pr_err(pr_fmt(fmt), ##__VA_ARGS__);       \
 		if ((vq)->error_ctx)                               \
 				eventfd_signal((vq)->error_ctx, 1);\
 	} while (0)
@@ -255,6 +256,8 @@  static inline void vhost_vq_set_backend(struct vhost_virtqueue *vq,
 					void *private_data)
 {
 	vq->private_data = private_data;
+	vq->ndescs = 0;
+	vq->first_desc = 0;
 }
 
 /**