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