diff mbox

[v5,13/13] mm: Clear shrinker bit if there are no objects related to memcg

Message ID 152594605549.22949.16491037134168999424.stgit@localhost.localdomain (mailing list archive)
State New, archived
Headers show

Commit Message

Kirill Tkhai May 10, 2018, 9:54 a.m. UTC
To avoid further unneed calls of do_shrink_slab()
for shrinkers, which already do not have any charged
objects in a memcg, their bits have to be cleared.

This patch introduces a lockless mechanism to do that
without races without parallel list lru add. After
do_shrink_slab() returns SHRINK_EMPTY the first time,
we clear the bit and call it once again. Then we restore
the bit, if the new return value is different.

Note, that single smp_mb__after_atomic() in shrink_slab_memcg()
covers two situations:

1)list_lru_add()     shrink_slab_memcg
    list_add_tail()    for_each_set_bit() <--- read bit
                         do_shrink_slab() <--- missed list update (no barrier)
    <MB>                 <MB>
    set_bit()            do_shrink_slab() <--- seen list update

This situation, when the first do_shrink_slab() sees set bit,
but it doesn't see list update (i.e., race with the first element
queueing), is rare. So we don't add <MB> before the first call
of do_shrink_slab() instead of this to do not slow down generic
case. Also, it's need the second call as seen in below in (2).

2)list_lru_add()      shrink_slab_memcg()
    list_add_tail()     ...
    set_bit()           ...
  ...                   for_each_set_bit()
  do_shrink_slab()        do_shrink_slab()
    clear_bit()           ...
  ...                     ...
  list_lru_add()          ...
    list_add_tail()       clear_bit()
    <MB>                  <MB>
    set_bit()             do_shrink_slab()

The barriers guarantees, the second do_shrink_slab()
in the right side task sees list update if really
cleared the bit. This case is drawn in the code comment.

[Results/performance of the patchset]

After the whole patchset applied the below test shows signify
increase of performance:

$echo 1 > /sys/fs/cgroup/memory/memory.use_hierarchy
$mkdir /sys/fs/cgroup/memory/ct
$echo 4000M > /sys/fs/cgroup/memory/ct/memory.kmem.limit_in_bytes
    $for i in `seq 0 4000`; do mkdir /sys/fs/cgroup/memory/ct/$i; echo $$ > /sys/fs/cgroup/memory/ct/$i/cgroup.procs; mkdir -p s/$i; mount -t tmpfs $i s/$i; touch s/$i/file; done

Then, 5 sequential calls of drop caches:
$time echo 3 > /proc/sys/vm/drop_caches

1)Before:
0.00user 13.78system 0:13.78elapsed 99%CPU
0.00user 5.59system 0:05.60elapsed 99%CPU
0.00user 5.48system 0:05.48elapsed 99%CPU
0.00user 8.35system 0:08.35elapsed 99%CPU
0.00user 8.34system 0:08.35elapsed 99%CPU

2)After
0.00user 1.10system 0:01.10elapsed 99%CPU
0.00user 0.00system 0:00.01elapsed 64%CPU
0.00user 0.01system 0:00.01elapsed 82%CPU
0.00user 0.00system 0:00.01elapsed 64%CPU
0.00user 0.01system 0:00.01elapsed 82%CPU

The results show the performance increases at least in 548 times.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
---
 include/linux/memcontrol.h |    2 ++
 mm/vmscan.c                |   19 +++++++++++++++++--
 2 files changed, 19 insertions(+), 2 deletions(-)

Comments

Vladimir Davydov May 15, 2018, 5:59 a.m. UTC | #1
On Thu, May 10, 2018 at 12:54:15PM +0300, Kirill Tkhai wrote:
> To avoid further unneed calls of do_shrink_slab()
> for shrinkers, which already do not have any charged
> objects in a memcg, their bits have to be cleared.
> 
> This patch introduces a lockless mechanism to do that
> without races without parallel list lru add. After
> do_shrink_slab() returns SHRINK_EMPTY the first time,
> we clear the bit and call it once again. Then we restore
> the bit, if the new return value is different.
> 
> Note, that single smp_mb__after_atomic() in shrink_slab_memcg()
> covers two situations:
> 
> 1)list_lru_add()     shrink_slab_memcg
>     list_add_tail()    for_each_set_bit() <--- read bit
>                          do_shrink_slab() <--- missed list update (no barrier)
>     <MB>                 <MB>
>     set_bit()            do_shrink_slab() <--- seen list update
> 
> This situation, when the first do_shrink_slab() sees set bit,
> but it doesn't see list update (i.e., race with the first element
> queueing), is rare. So we don't add <MB> before the first call
> of do_shrink_slab() instead of this to do not slow down generic
> case. Also, it's need the second call as seen in below in (2).
> 
> 2)list_lru_add()      shrink_slab_memcg()
>     list_add_tail()     ...
>     set_bit()           ...
>   ...                   for_each_set_bit()
>   do_shrink_slab()        do_shrink_slab()
>     clear_bit()           ...
>   ...                     ...
>   list_lru_add()          ...
>     list_add_tail()       clear_bit()
>     <MB>                  <MB>
>     set_bit()             do_shrink_slab()
> 
> The barriers guarantees, the second do_shrink_slab()
> in the right side task sees list update if really
> cleared the bit. This case is drawn in the code comment.
> 
> [Results/performance of the patchset]
> 
> After the whole patchset applied the below test shows signify
> increase of performance:
> 
> $echo 1 > /sys/fs/cgroup/memory/memory.use_hierarchy
> $mkdir /sys/fs/cgroup/memory/ct
> $echo 4000M > /sys/fs/cgroup/memory/ct/memory.kmem.limit_in_bytes
>     $for i in `seq 0 4000`; do mkdir /sys/fs/cgroup/memory/ct/$i; echo $$ > /sys/fs/cgroup/memory/ct/$i/cgroup.procs; mkdir -p s/$i; mount -t tmpfs $i s/$i; touch s/$i/file; done
> 
> Then, 5 sequential calls of drop caches:
> $time echo 3 > /proc/sys/vm/drop_caches
> 
> 1)Before:
> 0.00user 13.78system 0:13.78elapsed 99%CPU
> 0.00user 5.59system 0:05.60elapsed 99%CPU
> 0.00user 5.48system 0:05.48elapsed 99%CPU
> 0.00user 8.35system 0:08.35elapsed 99%CPU
> 0.00user 8.34system 0:08.35elapsed 99%CPU
> 
> 2)After
> 0.00user 1.10system 0:01.10elapsed 99%CPU
> 0.00user 0.00system 0:00.01elapsed 64%CPU
> 0.00user 0.01system 0:00.01elapsed 82%CPU
> 0.00user 0.00system 0:00.01elapsed 64%CPU
> 0.00user 0.01system 0:00.01elapsed 82%CPU
> 
> The results show the performance increases at least in 548 times.
> 
> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
> ---
>  include/linux/memcontrol.h |    2 ++
>  mm/vmscan.c                |   19 +++++++++++++++++--
>  2 files changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 436691a66500..82c0bf2d0579 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -1283,6 +1283,8 @@ static inline void memcg_set_shrinker_bit(struct mem_cgroup *memcg, int nid, int
>  
>  		rcu_read_lock();
>  		map = MEMCG_SHRINKER_MAP(memcg, nid);
> +		/* Pairs with smp mb in shrink_slab() */
> +		smp_mb__before_atomic();
>  		set_bit(nr, map->map);
>  		rcu_read_unlock();
>  	}
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 7b0075612d73..189b163bef4a 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -586,8 +586,23 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
>  			continue;
>  
>  		ret = do_shrink_slab(&sc, shrinker, priority);
> -		if (ret == SHRINK_EMPTY)
> -			ret = 0;
> +		if (ret == SHRINK_EMPTY) {
> +			clear_bit(i, map->map);
> +			/*
> +			 * Pairs with mb in memcg_set_shrinker_bit():
> +			 *
> +			 * list_lru_add()     shrink_slab_memcg()
> +			 *   list_add_tail()    clear_bit()
> +			 *   <MB>               <MB>
> +			 *   set_bit()          do_shrink_slab()
> +			 */

Please improve the comment so that it isn't just a diagram.

> +			smp_mb__after_atomic();
> +			ret = do_shrink_slab(&sc, shrinker, priority);
> +			if (ret == SHRINK_EMPTY)
> +				ret = 0;
> +			else
> +				memcg_set_shrinker_bit(memcg, nid, i);
> +		}
>  		freed += ret;
>  
>  		if (rwsem_is_contended(&shrinker_rwsem)) {
>
Kirill Tkhai May 15, 2018, 8:55 a.m. UTC | #2
On 15.05.2018 08:59, Vladimir Davydov wrote:
> On Thu, May 10, 2018 at 12:54:15PM +0300, Kirill Tkhai wrote:
>> To avoid further unneed calls of do_shrink_slab()
>> for shrinkers, which already do not have any charged
>> objects in a memcg, their bits have to be cleared.
>>
>> This patch introduces a lockless mechanism to do that
>> without races without parallel list lru add. After
>> do_shrink_slab() returns SHRINK_EMPTY the first time,
>> we clear the bit and call it once again. Then we restore
>> the bit, if the new return value is different.
>>
>> Note, that single smp_mb__after_atomic() in shrink_slab_memcg()
>> covers two situations:
>>
>> 1)list_lru_add()     shrink_slab_memcg
>>     list_add_tail()    for_each_set_bit() <--- read bit
>>                          do_shrink_slab() <--- missed list update (no barrier)
>>     <MB>                 <MB>
>>     set_bit()            do_shrink_slab() <--- seen list update
>>
>> This situation, when the first do_shrink_slab() sees set bit,
>> but it doesn't see list update (i.e., race with the first element
>> queueing), is rare. So we don't add <MB> before the first call
>> of do_shrink_slab() instead of this to do not slow down generic
>> case. Also, it's need the second call as seen in below in (2).
>>
>> 2)list_lru_add()      shrink_slab_memcg()
>>     list_add_tail()     ...
>>     set_bit()           ...
>>   ...                   for_each_set_bit()
>>   do_shrink_slab()        do_shrink_slab()
>>     clear_bit()           ...
>>   ...                     ...
>>   list_lru_add()          ...
>>     list_add_tail()       clear_bit()
>>     <MB>                  <MB>
>>     set_bit()             do_shrink_slab()
>>
>> The barriers guarantees, the second do_shrink_slab()
>> in the right side task sees list update if really
>> cleared the bit. This case is drawn in the code comment.
>>
>> [Results/performance of the patchset]
>>
>> After the whole patchset applied the below test shows signify
>> increase of performance:
>>
>> $echo 1 > /sys/fs/cgroup/memory/memory.use_hierarchy
>> $mkdir /sys/fs/cgroup/memory/ct
>> $echo 4000M > /sys/fs/cgroup/memory/ct/memory.kmem.limit_in_bytes
>>     $for i in `seq 0 4000`; do mkdir /sys/fs/cgroup/memory/ct/$i; echo $$ > /sys/fs/cgroup/memory/ct/$i/cgroup.procs; mkdir -p s/$i; mount -t tmpfs $i s/$i; touch s/$i/file; done
>>
>> Then, 5 sequential calls of drop caches:
>> $time echo 3 > /proc/sys/vm/drop_caches
>>
>> 1)Before:
>> 0.00user 13.78system 0:13.78elapsed 99%CPU
>> 0.00user 5.59system 0:05.60elapsed 99%CPU
>> 0.00user 5.48system 0:05.48elapsed 99%CPU
>> 0.00user 8.35system 0:08.35elapsed 99%CPU
>> 0.00user 8.34system 0:08.35elapsed 99%CPU
>>
>> 2)After
>> 0.00user 1.10system 0:01.10elapsed 99%CPU
>> 0.00user 0.00system 0:00.01elapsed 64%CPU
>> 0.00user 0.01system 0:00.01elapsed 82%CPU
>> 0.00user 0.00system 0:00.01elapsed 64%CPU
>> 0.00user 0.01system 0:00.01elapsed 82%CPU
>>
>> The results show the performance increases at least in 548 times.
>>
>> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
>> ---
>>  include/linux/memcontrol.h |    2 ++
>>  mm/vmscan.c                |   19 +++++++++++++++++--
>>  2 files changed, 19 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index 436691a66500..82c0bf2d0579 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -1283,6 +1283,8 @@ static inline void memcg_set_shrinker_bit(struct mem_cgroup *memcg, int nid, int
>>  
>>  		rcu_read_lock();
>>  		map = MEMCG_SHRINKER_MAP(memcg, nid);
>> +		/* Pairs with smp mb in shrink_slab() */
>> +		smp_mb__before_atomic();
>>  		set_bit(nr, map->map);
>>  		rcu_read_unlock();
>>  	}
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 7b0075612d73..189b163bef4a 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -586,8 +586,23 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
>>  			continue;
>>  
>>  		ret = do_shrink_slab(&sc, shrinker, priority);
>> -		if (ret == SHRINK_EMPTY)
>> -			ret = 0;
>> +		if (ret == SHRINK_EMPTY) {
>> +			clear_bit(i, map->map);
>> +			/*
>> +			 * Pairs with mb in memcg_set_shrinker_bit():
>> +			 *
>> +			 * list_lru_add()     shrink_slab_memcg()
>> +			 *   list_add_tail()    clear_bit()
>> +			 *   <MB>               <MB>
>> +			 *   set_bit()          do_shrink_slab()
>> +			 */
> 
> Please improve the comment so that it isn't just a diagram.

Please, say, which comment you want to see here.

>> +			smp_mb__after_atomic();
>> +			ret = do_shrink_slab(&sc, shrinker, priority);
>> +			if (ret == SHRINK_EMPTY)
>> +				ret = 0;
>> +			else
>> +				memcg_set_shrinker_bit(memcg, nid, i);
>> +		}
>>  		freed += ret;
>>  
>>  		if (rwsem_is_contended(&shrinker_rwsem)) {
>>
Vladimir Davydov May 17, 2018, 4:49 a.m. UTC | #3
On Tue, May 15, 2018 at 11:55:04AM +0300, Kirill Tkhai wrote:
> >> @@ -586,8 +586,23 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
> >>  			continue;
> >>  
> >>  		ret = do_shrink_slab(&sc, shrinker, priority);
> >> -		if (ret == SHRINK_EMPTY)
> >> -			ret = 0;
> >> +		if (ret == SHRINK_EMPTY) {
> >> +			clear_bit(i, map->map);
> >> +			/*
> >> +			 * Pairs with mb in memcg_set_shrinker_bit():
> >> +			 *
> >> +			 * list_lru_add()     shrink_slab_memcg()
> >> +			 *   list_add_tail()    clear_bit()
> >> +			 *   <MB>               <MB>
> >> +			 *   set_bit()          do_shrink_slab()
> >> +			 */
> > 
> > Please improve the comment so that it isn't just a diagram.
> 
> Please, say, which comment you want to see here.

I want the reader to understand why we need to invoke the shrinker twice
if it returns SHRINK_EMPTY. The diagram doesn't really help here IMO. So
I'd write Something like this:

	ret = do_shrink_slab(&sc, shrinker, priority);
	if (ret == SHRINK_EMPTY) {
		clear_bit(i, map->map);
		/*
		 * After the shrinker reported that it had no objects to free,
		 * but before we cleared the corresponding bit in the memcg
		 * shrinker map, a new object might have been added. To make
		 * sure, we have the bit set in this case, we invoke the
		 * shrinker one more time and re-set the bit if it reports that
		 * it is not empty anymore. The memory barrier here pairs with
		 * the barrier in memcg_set_shrinker_bit():
		 *
		 * list_lru_add()     shrink_slab_memcg()
		 *   list_add_tail()    clear_bit()
		 *   <MB>               <MB>
		 *   set_bit()          do_shrink_slab()
		 */
		smp_mb__after_atomic();
		ret = do_shrink_slab(&sc, shrinker, priority);
			if (ret == SHRINK_EMPTY)
				ret = 0;
			else
				memcg_set_shrinker_bit(memcg, nid, i);
diff mbox

Patch

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 436691a66500..82c0bf2d0579 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1283,6 +1283,8 @@  static inline void memcg_set_shrinker_bit(struct mem_cgroup *memcg, int nid, int
 
 		rcu_read_lock();
 		map = MEMCG_SHRINKER_MAP(memcg, nid);
+		/* Pairs with smp mb in shrink_slab() */
+		smp_mb__before_atomic();
 		set_bit(nr, map->map);
 		rcu_read_unlock();
 	}
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 7b0075612d73..189b163bef4a 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -586,8 +586,23 @@  static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
 			continue;
 
 		ret = do_shrink_slab(&sc, shrinker, priority);
-		if (ret == SHRINK_EMPTY)
-			ret = 0;
+		if (ret == SHRINK_EMPTY) {
+			clear_bit(i, map->map);
+			/*
+			 * Pairs with mb in memcg_set_shrinker_bit():
+			 *
+			 * list_lru_add()     shrink_slab_memcg()
+			 *   list_add_tail()    clear_bit()
+			 *   <MB>               <MB>
+			 *   set_bit()          do_shrink_slab()
+			 */
+			smp_mb__after_atomic();
+			ret = do_shrink_slab(&sc, shrinker, priority);
+			if (ret == SHRINK_EMPTY)
+				ret = 0;
+			else
+				memcg_set_shrinker_bit(memcg, nid, i);
+		}
 		freed += ret;
 
 		if (rwsem_is_contended(&shrinker_rwsem)) {