On Mon, Apr 23, 2018 at 01:01:08PM +0300, Kirill Tkhai wrote: > 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. Fair enough. I guess this is the best way to go after all. Please try to improve the comment so that it isn't just a pure diagram. Also, please prefix all memcg-related function names (such as set_shrinker_bit) with memcg_ (or mem_cgroup_) in this and all other patches.