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Subject: Re: [PATCH 10/10] mm: Clear shrinker bit if there are no objects
 related to memcg
To:     Vladimir Davydov <vdavydov.dev@gmail.com>
Cc:     viro@zeniv.linux.org.uk, hannes@cmpxchg.org, mhocko@kernel.org,
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        linux@roeck-us.net, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, willy@infradead.org
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 <20180324203312.b2whjgadm7gwby3v@esperanza>
From:   Kirill Tkhai <ktkhai@virtuozzo.com>
Message-ID: <f396088e-82e9-1a49-3ce1-cafc61b6ff90@virtuozzo.com>
Date:   Mon, 26 Mar 2018 18:37:13 +0300
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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