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SubjectRe: [PATCH 10/10] mm: Clear shrinker bit if there are no objects related to memcg
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On 24.03.2018 23:33, Vladimir Davydov wrote:
> On Wed, Mar 21, 2018 at 04:23:01PM +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 new return value SHRINK_EMPTY,
>> which will be used in case of there is no charged
>> objects in shrinker. We can't use 0 instead of that,
>> as a shrinker may return 0, when it has very small
>> amount of objects.
>>
>> To prevent race with parallel list lru add, we call
>> do_shrink_slab() once again, after the bit is cleared.
>> So, if there is a new object, we never miss it, and
>> the bit will be restored again.
>>
>> The below test shows significant performance growths
>> after using the patchset:
>>
>> $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 4 drop_caches:
>> $time echo 3 > /proc/sys/vm/drop_caches
>>
>> Times of drop_caches:
>>
>> *Before (4 iterations)*
>> 0.00user 6.80system 0:06.82elapsed 99%CPU
>> 0.00user 4.61system 0:04.62elapsed 99%CPU
>> 0.00user 4.61system 0:04.61elapsed 99%CPU
>> 0.00user 4.61system 0:04.61elapsed 99%CPU
>>
>> *After (4 iterations)*
>> 0.00user 0.93system 0:00.94elapsed 99%CPU
>> 0.00user 0.00system 0:00.01elapsed 80%CPU
>> 0.00user 0.00system 0:00.01elapsed 80%CPU
>> 0.00user 0.00system 0:00.01elapsed 81%CPU
>>
>> 4.61s/0.01s = 461 times faster.
>>
>> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
>> ---
>> fs/super.c | 3 +++
>> include/linux/shrinker.h | 1 +
>> mm/vmscan.c | 21 ++++++++++++++++++---
>> mm/workingset.c | 3 +++
>> 4 files changed, 25 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
>> index 24aeed1bc332..b23180deb928 100644
>> --- a/include/linux/shrinker.h
>> +++ b/include/linux/shrinker.h
>> @@ -34,6 +34,7 @@ struct shrink_control {
>> };
>>
>> #define SHRINK_STOP (~0UL)
>> +#define SHRINK_EMPTY (~0UL - 1)
>
> Please update the comment below accordingly.

Ok

>> /*
>> * A callback you can register to apply pressure to ageable caches.
>> *
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index e1fd16bc7a9b..1fc05e8bde04 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -387,6 +387,7 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int nr)
>> {
>> struct shrinkers_map *map = SHRINKERS_MAP(memcg);
>>
>> + smp_mb__before_atomic(); /* Pairs with mb in shrink_slab() */
>
> I don't understand the purpose of this barrier. Please add a comment
> explaining why you need it.

Ok

>> set_bit(nr, map->map[nid]);
>> }
>> #else /* CONFIG_MEMCG && !CONFIG_SLOB */
>> @@ -568,8 +569,8 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>> long scanned = 0, next_deferred;
>>
>> freeable = shrinker->count_objects(shrinker, shrinkctl);
>> - if (freeable == 0)
>> - return 0;
>> + if (freeable == 0 || freeable == SHRINK_EMPTY)
>> + return freeable;
>>
>> /*
>> * copy the current shrinker scan count into a local variable
>> @@ -708,6 +709,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>> #if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB)
>> if (!memcg_kmem_enabled() || memcg) {
>> struct shrinkers_map *map;
>> + unsigned long ret;
>> int i;
>>
>> map = rcu_dereference_protected(SHRINKERS_MAP(memcg), true);
>> @@ -724,7 +726,20 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>> clear_bit(i, map->map[nid]);
>> continue;
>> }
>> - freed += do_shrink_slab(&sc, shrinker, priority);
>> + if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
>> + sc.nid = 0;
>
> Hmm, if my memory doesn't fail, in the previous patch you added a BUG_ON
> ensuring that a memcg-aware shrinker must also be numa-aware while here
> you still check it. Please remove the BUG_ON or remove this check.
> Better remove the BUG_ON, because a memcg-aware shrinker doesn't have to
> be numa-aware.

Really, we do not introduce new limitations, so it's need to just remove the BUG_ON.
Will do in v2.

>
>> + ret = do_shrink_slab(&sc, shrinker, priority);
>> + if (ret == SHRINK_EMPTY) {
>
> do_shrink_slab() is also called for memcg-unaware shrinkers, you should
> probably handle SHRINK_EMPTY there as well.

Ok, it looks like we just need to return 0 instead of SHRINK_EMPTY in such cases.

>> + clear_bit(i, map->map[nid]);
>> + /* pairs with mb in set_shrinker_bit() */
>> + smp_mb__after_atomic();
>> + ret = do_shrink_slab(&sc, shrinker, priority);
>> + if (ret == SHRINK_EMPTY)
>> + ret = 0;
>> + else
>> + set_bit(i, map->map[nid]);
>
> Well, that's definitely a tricky part and hence needs a good comment.
>
> Anyway, it would be great if we could simplify this part somehow.

Ok, I'll add some cleanup preparations for that.

>> + }
>> + freed += ret;
>>
>> if (rwsem_is_contended(&shrinker_rwsem)) {
>> freed = freed ? : 1;

Thanks,
Kirill

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