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@xxxxxxxxxxxxx>
>> ---
>> 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
