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Subject[PATCH v5 13/13] mm: Clear shrinker bit if there are no objects related to memcg
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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, 5 sequential calls of drop caches:
$time echo 3 > /proc/sys/vm/drop_caches

1)Before:
0.00user 13.78system 0:13.78elapsed 99%CPU
0.00user 5.59system 0:05.60elapsed 99%CPU
0.00user 5.48system 0:05.48elapsed 99%CPU
0.00user 8.35system 0:08.35elapsed 99%CPU
0.00user 8.34system 0:08.35elapsed 99%CPU

2)After
0.00user 1.10system 0:01.10elapsed 99%CPU
0.00user 0.00system 0:00.01elapsed 64%CPU
0.00user 0.01system 0:00.01elapsed 82%CPU
0.00user 0.00system 0:00.01elapsed 64%CPU
0.00user 0.01system 0:00.01elapsed 82%CPU

The results show the performance increases at least in 548 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 436691a66500..82c0bf2d0579 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1283,6 +1283,8 @@ static inline void memcg_set_shrinker_bit(struct mem_cgroup *memcg, int nid, int

rcu_read_lock();
map = MEMCG_SHRINKER_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 7b0075612d73..189b163bef4a 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -586,8 +586,23 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
continue;

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 memcg_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
+ memcg_set_shrinker_bit(memcg, nid, i);
+ }
freed += ret;

if (rwsem_is_contended(&shrinker_rwsem)) {
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