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