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Subject: Re: [PATCH v2 12/12] mm: Clear shrinker bit if there are no objects
 related to memcg
To:     Vladimir Davydov <vdavydov.dev@gmail.com>
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        viro@zeniv.linux.org.uk, hannes@cmpxchg.org, mhocko@kernel.org,
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From:   Kirill Tkhai <ktkhai@virtuozzo.com>
Message-ID: <17b76fd4-ce80-50cf-6149-1f3908081ae7@virtuozzo.com>
Date:   Mon, 23 Apr 2018 13:01:08 +0300
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On 22.04.2018 21:21, Vladimir Davydov wrote:
> On Tue, Apr 17, 2018 at 09:54:51PM +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, 4 sequential calls of drop caches:
>> $time echo 3 > /proc/sys/vm/drop_caches
>>
>> 1)Before:
>> 0.00user 8.99system 0:08.99elapsed 99%CPU
>> 0.00user 5.97system 0:05.97elapsed 100%CPU
>> 0.00user 5.97system 0:05.97elapsed 100%CPU
>> 0.00user 5.85system 0:05.85elapsed 100%CPU
>>
>> 2)After
>> 0.00user 1.11system 0:01.12elapsed 99%CPU
>> 0.00user 0.00system 0:00.00elapsed 100%CPU
>> 0.00user 0.00system 0:00.00elapsed 100%CPU
>> 0.00user 0.00system 0:00.00elapsed 100%CPU
>>
>> Even if we round 0:00.00 up to 0:00.01, the results shows
>> the performance increases at least in 585 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 e1c1fa8e417a..1c5c68550e2f 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -1245,6 +1245,8 @@ static inline void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int nr)
>>  
>>  		rcu_read_lock();
>>  		map = SHRINKERS_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 3be9b4d81c13..a8733bc5377b 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -579,8 +579,23 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
>>  		}
>>  
>>  		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 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
>> +				set_shrinker_bit(memcg, nid, i);
>> +		}
> 
> This is mind-boggling. Are there any alternatives? For instance, can't
> we clear the bit in list_lru_del, when we hold the list lock?

Since a single shrinker may iterate over several lru lists, we can't do that.
Otherwise, we would have to probe another shrinker's lru list from a lru list,
which became empty in list_lru_del().

The solution I suggested, is generic, and it does not depend on low-level
structure type, used by shrinker. This even doesn't have to be a lru list.

>>  		freed += ret;
>>  
>>  		if (rwsem_is_contended(&shrinker_rwsem)) {

Kirill