diff mbox series

[v4,5/5] mm/memcg: Improve refill_obj_stock() performance

Message ID 20210419000032.5432-6-longman@redhat.com (mailing list archive)
State New
Headers show
Series mm/memcg: Reduce kmemcache memory accounting overhead | expand

Commit Message

Waiman Long April 19, 2021, midnight UTC
There are two issues with the current refill_obj_stock() code. First of
all, when nr_bytes reaches over PAGE_SIZE, it calls drain_obj_stock() to
atomically flush out remaining bytes to obj_cgroup, clear cached_objcg
and do a obj_cgroup_put(). It is likely that the same obj_cgroup will
be used again which leads to another call to drain_obj_stock() and
obj_cgroup_get() as well as atomically retrieve the available byte from
obj_cgroup. That is costly. Instead, we should just uncharge the excess
pages, reduce the stock bytes and be done with it. The drain_obj_stock()
function should only be called when obj_cgroup changes.

Secondly, when charging an object of size not less than a page in
obj_cgroup_charge(), it is possible that the remaining bytes to be
refilled to the stock will overflow a page and cause refill_obj_stock()
to uncharge 1 page. To avoid the additional uncharge in this case,
a new overfill flag is added to refill_obj_stock() which will be set
when called from obj_cgroup_charge().

Signed-off-by: Waiman Long <longman@redhat.com>
---
 mm/memcontrol.c | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

Comments

Muchun Song April 19, 2021, 6:06 a.m. UTC | #1
On Mon, Apr 19, 2021 at 8:01 AM Waiman Long <longman@redhat.com> wrote:
>
> There are two issues with the current refill_obj_stock() code. First of
> all, when nr_bytes reaches over PAGE_SIZE, it calls drain_obj_stock() to
> atomically flush out remaining bytes to obj_cgroup, clear cached_objcg
> and do a obj_cgroup_put(). It is likely that the same obj_cgroup will
> be used again which leads to another call to drain_obj_stock() and
> obj_cgroup_get() as well as atomically retrieve the available byte from
> obj_cgroup. That is costly. Instead, we should just uncharge the excess
> pages, reduce the stock bytes and be done with it. The drain_obj_stock()
> function should only be called when obj_cgroup changes.
>
> Secondly, when charging an object of size not less than a page in
> obj_cgroup_charge(), it is possible that the remaining bytes to be
> refilled to the stock will overflow a page and cause refill_obj_stock()
> to uncharge 1 page. To avoid the additional uncharge in this case,
> a new overfill flag is added to refill_obj_stock() which will be set
> when called from obj_cgroup_charge().
>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  mm/memcontrol.c | 23 +++++++++++++++++------
>  1 file changed, 17 insertions(+), 6 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index a6dd18f6d8a8..d13961352eef 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3357,23 +3357,34 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
>         return false;
>  }
>
> -static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
> +static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
> +                            bool overfill)
>  {
>         unsigned long flags;
>         struct obj_stock *stock = get_obj_stock(&flags);
> +       unsigned int nr_pages = 0;
>
>         if (stock->cached_objcg != objcg) { /* reset if necessary */
> -               drain_obj_stock(stock);
> +               if (stock->cached_objcg)
> +                       drain_obj_stock(stock);
>                 obj_cgroup_get(objcg);
>                 stock->cached_objcg = objcg;
>                 stock->nr_bytes = atomic_xchg(&objcg->nr_charged_bytes, 0);
>         }
>         stock->nr_bytes += nr_bytes;
>
> -       if (stock->nr_bytes > PAGE_SIZE)
> -               drain_obj_stock(stock);
> +       if (!overfill && (stock->nr_bytes > PAGE_SIZE)) {
> +               nr_pages = stock->nr_bytes >> PAGE_SHIFT;
> +               stock->nr_bytes &= (PAGE_SIZE - 1);
> +       }
>
>         put_obj_stock(flags);
> +
> +       if (nr_pages) {
> +               rcu_read_lock();
> +               __memcg_kmem_uncharge(obj_cgroup_memcg(objcg), nr_pages);
> +               rcu_read_unlock();
> +       }

It is not safe to call __memcg_kmem_uncharge() under rcu lock
and without holding a reference to memcg. More details can refer
to the following link.

https://lore.kernel.org/linux-mm/20210319163821.20704-2-songmuchun@bytedance.com/

In the above patchset, we introduce obj_cgroup_uncharge_pages to
uncharge some pages from object cgroup. You can use this safe
API.

Thanks.

>  }
>
>  int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
> @@ -3410,7 +3421,7 @@ int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
>
>         ret = __memcg_kmem_charge(memcg, gfp, nr_pages);
>         if (!ret && nr_bytes)
> -               refill_obj_stock(objcg, PAGE_SIZE - nr_bytes);
> +               refill_obj_stock(objcg, PAGE_SIZE - nr_bytes, true);
>
>         css_put(&memcg->css);
>         return ret;
> @@ -3418,7 +3429,7 @@ int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
>
>  void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size)
>  {
> -       refill_obj_stock(objcg, size);
> +       refill_obj_stock(objcg, size, false);
>  }
>
>  #endif /* CONFIG_MEMCG_KMEM */
> --
> 2.18.1
>
Shakeel Butt April 19, 2021, 3 p.m. UTC | #2
On Sun, Apr 18, 2021 at 11:07 PM Muchun Song <songmuchun@bytedance.com> wrote:
>
> On Mon, Apr 19, 2021 at 8:01 AM Waiman Long <longman@redhat.com> wrote:
> >
> > There are two issues with the current refill_obj_stock() code. First of
> > all, when nr_bytes reaches over PAGE_SIZE, it calls drain_obj_stock() to
> > atomically flush out remaining bytes to obj_cgroup, clear cached_objcg
> > and do a obj_cgroup_put(). It is likely that the same obj_cgroup will
> > be used again which leads to another call to drain_obj_stock() and
> > obj_cgroup_get() as well as atomically retrieve the available byte from
> > obj_cgroup. That is costly. Instead, we should just uncharge the excess
> > pages, reduce the stock bytes and be done with it. The drain_obj_stock()
> > function should only be called when obj_cgroup changes.
> >
> > Secondly, when charging an object of size not less than a page in
> > obj_cgroup_charge(), it is possible that the remaining bytes to be
> > refilled to the stock will overflow a page and cause refill_obj_stock()
> > to uncharge 1 page. To avoid the additional uncharge in this case,
> > a new overfill flag is added to refill_obj_stock() which will be set
> > when called from obj_cgroup_charge().
> >
> > Signed-off-by: Waiman Long <longman@redhat.com>
> > ---
> >  mm/memcontrol.c | 23 +++++++++++++++++------
> >  1 file changed, 17 insertions(+), 6 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index a6dd18f6d8a8..d13961352eef 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -3357,23 +3357,34 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
> >         return false;
> >  }
> >
> > -static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
> > +static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
> > +                            bool overfill)
> >  {
> >         unsigned long flags;
> >         struct obj_stock *stock = get_obj_stock(&flags);
> > +       unsigned int nr_pages = 0;
> >
> >         if (stock->cached_objcg != objcg) { /* reset if necessary */
> > -               drain_obj_stock(stock);
> > +               if (stock->cached_objcg)
> > +                       drain_obj_stock(stock);
> >                 obj_cgroup_get(objcg);
> >                 stock->cached_objcg = objcg;
> >                 stock->nr_bytes = atomic_xchg(&objcg->nr_charged_bytes, 0);
> >         }
> >         stock->nr_bytes += nr_bytes;
> >
> > -       if (stock->nr_bytes > PAGE_SIZE)
> > -               drain_obj_stock(stock);
> > +       if (!overfill && (stock->nr_bytes > PAGE_SIZE)) {
> > +               nr_pages = stock->nr_bytes >> PAGE_SHIFT;
> > +               stock->nr_bytes &= (PAGE_SIZE - 1);
> > +       }
> >
> >         put_obj_stock(flags);
> > +
> > +       if (nr_pages) {
> > +               rcu_read_lock();
> > +               __memcg_kmem_uncharge(obj_cgroup_memcg(objcg), nr_pages);
> > +               rcu_read_unlock();
> > +       }
>
> It is not safe to call __memcg_kmem_uncharge() under rcu lock
> and without holding a reference to memcg. More details can refer
> to the following link.
>
> https://lore.kernel.org/linux-mm/20210319163821.20704-2-songmuchun@bytedance.com/
>
> In the above patchset, we introduce obj_cgroup_uncharge_pages to
> uncharge some pages from object cgroup. You can use this safe
> API.
>

I would recommend just rebase the patch series over the latest mm tree.
Waiman Long April 19, 2021, 3:19 p.m. UTC | #3
On 4/19/21 11:00 AM, Shakeel Butt wrote:
> On Sun, Apr 18, 2021 at 11:07 PM Muchun Song <songmuchun@bytedance.com> wrote:
>> On Mon, Apr 19, 2021 at 8:01 AM Waiman Long <longman@redhat.com> wrote:
>>> There are two issues with the current refill_obj_stock() code. First of
>>> all, when nr_bytes reaches over PAGE_SIZE, it calls drain_obj_stock() to
>>> atomically flush out remaining bytes to obj_cgroup, clear cached_objcg
>>> and do a obj_cgroup_put(). It is likely that the same obj_cgroup will
>>> be used again which leads to another call to drain_obj_stock() and
>>> obj_cgroup_get() as well as atomically retrieve the available byte from
>>> obj_cgroup. That is costly. Instead, we should just uncharge the excess
>>> pages, reduce the stock bytes and be done with it. The drain_obj_stock()
>>> function should only be called when obj_cgroup changes.
>>>
>>> Secondly, when charging an object of size not less than a page in
>>> obj_cgroup_charge(), it is possible that the remaining bytes to be
>>> refilled to the stock will overflow a page and cause refill_obj_stock()
>>> to uncharge 1 page. To avoid the additional uncharge in this case,
>>> a new overfill flag is added to refill_obj_stock() which will be set
>>> when called from obj_cgroup_charge().
>>>
>>> Signed-off-by: Waiman Long <longman@redhat.com>
>>> ---
>>>   mm/memcontrol.c | 23 +++++++++++++++++------
>>>   1 file changed, 17 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>> index a6dd18f6d8a8..d13961352eef 100644
>>> --- a/mm/memcontrol.c
>>> +++ b/mm/memcontrol.c
>>> @@ -3357,23 +3357,34 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
>>>          return false;
>>>   }
>>>
>>> -static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
>>> +static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
>>> +                            bool overfill)
>>>   {
>>>          unsigned long flags;
>>>          struct obj_stock *stock = get_obj_stock(&flags);
>>> +       unsigned int nr_pages = 0;
>>>
>>>          if (stock->cached_objcg != objcg) { /* reset if necessary */
>>> -               drain_obj_stock(stock);
>>> +               if (stock->cached_objcg)
>>> +                       drain_obj_stock(stock);
>>>                  obj_cgroup_get(objcg);
>>>                  stock->cached_objcg = objcg;
>>>                  stock->nr_bytes = atomic_xchg(&objcg->nr_charged_bytes, 0);
>>>          }
>>>          stock->nr_bytes += nr_bytes;
>>>
>>> -       if (stock->nr_bytes > PAGE_SIZE)
>>> -               drain_obj_stock(stock);
>>> +       if (!overfill && (stock->nr_bytes > PAGE_SIZE)) {
>>> +               nr_pages = stock->nr_bytes >> PAGE_SHIFT;
>>> +               stock->nr_bytes &= (PAGE_SIZE - 1);
>>> +       }
>>>
>>>          put_obj_stock(flags);
>>> +
>>> +       if (nr_pages) {
>>> +               rcu_read_lock();
>>> +               __memcg_kmem_uncharge(obj_cgroup_memcg(objcg), nr_pages);
>>> +               rcu_read_unlock();
>>> +       }
>> It is not safe to call __memcg_kmem_uncharge() under rcu lock
>> and without holding a reference to memcg. More details can refer
>> to the following link.
>>
>> https://lore.kernel.org/linux-mm/20210319163821.20704-2-songmuchun@bytedance.com/
>>
>> In the above patchset, we introduce obj_cgroup_uncharge_pages to
>> uncharge some pages from object cgroup. You can use this safe
>> API.
>>
> I would recommend just rebase the patch series over the latest mm tree.
>
I see, I will rebase it to the latest mm tree.

Thanks,
Longman
Waiman Long April 19, 2021, 3:56 p.m. UTC | #4
On 4/19/21 2:06 AM, Muchun Song wrote:
> On Mon, Apr 19, 2021 at 8:01 AM Waiman Long <longman@redhat.com> wrote:
>> There are two issues with the current refill_obj_stock() code. First of
>> all, when nr_bytes reaches over PAGE_SIZE, it calls drain_obj_stock() to
>> atomically flush out remaining bytes to obj_cgroup, clear cached_objcg
>> and do a obj_cgroup_put(). It is likely that the same obj_cgroup will
>> be used again which leads to another call to drain_obj_stock() and
>> obj_cgroup_get() as well as atomically retrieve the available byte from
>> obj_cgroup. That is costly. Instead, we should just uncharge the excess
>> pages, reduce the stock bytes and be done with it. The drain_obj_stock()
>> function should only be called when obj_cgroup changes.
>>
>> Secondly, when charging an object of size not less than a page in
>> obj_cgroup_charge(), it is possible that the remaining bytes to be
>> refilled to the stock will overflow a page and cause refill_obj_stock()
>> to uncharge 1 page. To avoid the additional uncharge in this case,
>> a new overfill flag is added to refill_obj_stock() which will be set
>> when called from obj_cgroup_charge().
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   mm/memcontrol.c | 23 +++++++++++++++++------
>>   1 file changed, 17 insertions(+), 6 deletions(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index a6dd18f6d8a8..d13961352eef 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -3357,23 +3357,34 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
>>          return false;
>>   }
>>
>> -static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
>> +static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
>> +                            bool overfill)
>>   {
>>          unsigned long flags;
>>          struct obj_stock *stock = get_obj_stock(&flags);
>> +       unsigned int nr_pages = 0;
>>
>>          if (stock->cached_objcg != objcg) { /* reset if necessary */
>> -               drain_obj_stock(stock);
>> +               if (stock->cached_objcg)
>> +                       drain_obj_stock(stock);
>>                  obj_cgroup_get(objcg);
>>                  stock->cached_objcg = objcg;
>>                  stock->nr_bytes = atomic_xchg(&objcg->nr_charged_bytes, 0);
>>          }
>>          stock->nr_bytes += nr_bytes;
>>
>> -       if (stock->nr_bytes > PAGE_SIZE)
>> -               drain_obj_stock(stock);
>> +       if (!overfill && (stock->nr_bytes > PAGE_SIZE)) {
>> +               nr_pages = stock->nr_bytes >> PAGE_SHIFT;
>> +               stock->nr_bytes &= (PAGE_SIZE - 1);
>> +       }
>>
>>          put_obj_stock(flags);
>> +
>> +       if (nr_pages) {
>> +               rcu_read_lock();
>> +               __memcg_kmem_uncharge(obj_cgroup_memcg(objcg), nr_pages);
>> +               rcu_read_unlock();
>> +       }
> It is not safe to call __memcg_kmem_uncharge() under rcu lock
> and without holding a reference to memcg. More details can refer
> to the following link.
>
> https://lore.kernel.org/linux-mm/20210319163821.20704-2-songmuchun@bytedance.com/
>
> In the above patchset, we introduce obj_cgroup_uncharge_pages to
> uncharge some pages from object cgroup. You can use this safe
> API.

Thanks for the comment. Will update my patch with call to 
obj_cgroup_uncharge_pages().

Cheers,
Longman
diff mbox series

Patch

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index a6dd18f6d8a8..d13961352eef 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3357,23 +3357,34 @@  static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
 	return false;
 }
 
-static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
+static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
+			     bool overfill)
 {
 	unsigned long flags;
 	struct obj_stock *stock = get_obj_stock(&flags);
+	unsigned int nr_pages = 0;
 
 	if (stock->cached_objcg != objcg) { /* reset if necessary */
-		drain_obj_stock(stock);
+		if (stock->cached_objcg)
+			drain_obj_stock(stock);
 		obj_cgroup_get(objcg);
 		stock->cached_objcg = objcg;
 		stock->nr_bytes = atomic_xchg(&objcg->nr_charged_bytes, 0);
 	}
 	stock->nr_bytes += nr_bytes;
 
-	if (stock->nr_bytes > PAGE_SIZE)
-		drain_obj_stock(stock);
+	if (!overfill && (stock->nr_bytes > PAGE_SIZE)) {
+		nr_pages = stock->nr_bytes >> PAGE_SHIFT;
+		stock->nr_bytes &= (PAGE_SIZE - 1);
+	}
 
 	put_obj_stock(flags);
+
+	if (nr_pages) {
+		rcu_read_lock();
+		__memcg_kmem_uncharge(obj_cgroup_memcg(objcg), nr_pages);
+		rcu_read_unlock();
+	}
 }
 
 int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
@@ -3410,7 +3421,7 @@  int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
 
 	ret = __memcg_kmem_charge(memcg, gfp, nr_pages);
 	if (!ret && nr_bytes)
-		refill_obj_stock(objcg, PAGE_SIZE - nr_bytes);
+		refill_obj_stock(objcg, PAGE_SIZE - nr_bytes, true);
 
 	css_put(&memcg->css);
 	return ret;
@@ -3418,7 +3429,7 @@  int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
 
 void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size)
 {
-	refill_obj_stock(objcg, size);
+	refill_obj_stock(objcg, size, false);
 }
 
 #endif /* CONFIG_MEMCG_KMEM */