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Message-ID: <152663307060.5308.7860175207287376342.stgit@localhost.localdomain>
Date: Fri, 18 May 2018 11:44:30 +0300
From: Kirill Tkhai <ktkhai@...tuozzo.com>
To: akpm@...ux-foundation.org, vdavydov.dev@...il.com,
shakeelb@...gle.com, viro@...iv.linux.org.uk, hannes@...xchg.org,
mhocko@...nel.org, ktkhai@...tuozzo.com, tglx@...utronix.de,
pombredanne@...b.com, stummala@...eaurora.org,
gregkh@...uxfoundation.org, sfr@...b.auug.org.au, guro@...com,
mka@...omium.org, penguin-kernel@...ove.SAKURA.ne.jp,
chris@...is-wilson.co.uk, longman@...hat.com, minchan@...nel.org,
ying.huang@...el.com, mgorman@...hsingularity.net, jbacik@...com,
linux@...ck-us.net, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, willy@...radead.org, lirongqing@...du.com,
aryabinin@...tuozzo.com
Subject: [PATCH v6 17/17] mm: Clear shrinker bit if there are no objects
related to memcg
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@...tuozzo.com>
---
include/linux/memcontrol.h | 2 ++
mm/vmscan.c | 25 +++++++++++++++++++++++--
2 files changed, 25 insertions(+), 2 deletions(-)
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index cd44c1fac22b..dcbb7e03501d 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1293,6 +1293,8 @@ static inline void memcg_set_shrinker_bit(struct mem_cgroup *memcg,
rcu_read_lock();
map = rcu_dereference(memcg->nodeinfo[nid]->shrinker_map);
+ /* Pairs with smp mb in shrink_slab() */
+ smp_mb__before_atomic();
set_bit(shrinker_id, map->map);
rcu_read_unlock();
}
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 6639d1e7b6a6..c99c248cdfcd 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -600,8 +600,29 @@ 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);
+ /*
+ * After the shrinker reported that it had no objects to free,
+ * but before we cleared the corresponding bit in the memcg
+ * shrinker map, a new object might have been added. To make
+ * sure, we have the bit set in this case, we invoke the
+ * shrinker one more time and re-set the bit if it reports that
+ * it is not empty anymore. The memory barrier here pairs with
+ * the barrier 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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