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    Subject[PATCH 10/10] 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 new return value SHRINK_EMPTY,
    which will be used in case of there is no charged
    objects in shrinker. We can't use 0 instead of that,
    as a shrinker may return 0, when it has very small
    amount of objects.

    To prevent race with parallel list lru add, we call
    do_shrink_slab() once again, after the bit is cleared.
    So, if there is a new object, we never miss it, and
    the bit will be restored again.

    The below test shows significant performance growths
    after using the patchset:

    $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 drop_caches:
    $time echo 3 > /proc/sys/vm/drop_caches

    Times of drop_caches:

    *Before (4 iterations)*
    0.00user 6.80system 0:06.82elapsed 99%CPU
    0.00user 4.61system 0:04.62elapsed 99%CPU
    0.00user 4.61system 0:04.61elapsed 99%CPU
    0.00user 4.61system 0:04.61elapsed 99%CPU

    *After (4 iterations)*
    0.00user 0.93system 0:00.94elapsed 99%CPU
    0.00user 0.00system 0:00.01elapsed 80%CPU
    0.00user 0.00system 0:00.01elapsed 80%CPU
    0.00user 0.00system 0:00.01elapsed 81%CPU

    4.61s/0.01s = 461 times faster.

    Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
    ---
    fs/super.c | 3 +++
    include/linux/shrinker.h | 1 +
    mm/vmscan.c | 21 ++++++++++++++++++---
    mm/workingset.c | 3 +++
    4 files changed, 25 insertions(+), 3 deletions(-)

    diff --git a/fs/super.c b/fs/super.c
    index 1f3dc4eab409..788ea4e6a40b 100644
    --- a/fs/super.c
    +++ b/fs/super.c
    @@ -133,6 +133,9 @@ static unsigned long super_cache_count(struct shrinker *shrink,
    total_objects += list_lru_shrink_count(&sb->s_dentry_lru, sc);
    total_objects += list_lru_shrink_count(&sb->s_inode_lru, sc);

    + if (!total_objects)
    + return SHRINK_EMPTY;
    +
    total_objects = vfs_pressure_ratio(total_objects);
    return total_objects;
    }
    diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
    index 24aeed1bc332..b23180deb928 100644
    --- a/include/linux/shrinker.h
    +++ b/include/linux/shrinker.h
    @@ -34,6 +34,7 @@ struct shrink_control {
    };

    #define SHRINK_STOP (~0UL)
    +#define SHRINK_EMPTY (~0UL - 1)
    /*
    * A callback you can register to apply pressure to ageable caches.
    *
    diff --git a/mm/vmscan.c b/mm/vmscan.c
    index e1fd16bc7a9b..1fc05e8bde04 100644
    --- a/mm/vmscan.c
    +++ b/mm/vmscan.c
    @@ -387,6 +387,7 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int nr)
    {
    struct shrinkers_map *map = SHRINKERS_MAP(memcg);

    + smp_mb__before_atomic(); /* Pairs with mb in shrink_slab() */
    set_bit(nr, map->map[nid]);
    }
    #else /* CONFIG_MEMCG && !CONFIG_SLOB */
    @@ -568,8 +569,8 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
    long scanned = 0, next_deferred;

    freeable = shrinker->count_objects(shrinker, shrinkctl);
    - if (freeable == 0)
    - return 0;
    + if (freeable == 0 || freeable == SHRINK_EMPTY)
    + return freeable;

    /*
    * copy the current shrinker scan count into a local variable
    @@ -708,6 +709,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
    #if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB)
    if (!memcg_kmem_enabled() || memcg) {
    struct shrinkers_map *map;
    + unsigned long ret;
    int i;

    map = rcu_dereference_protected(SHRINKERS_MAP(memcg), true);
    @@ -724,7 +726,20 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
    clear_bit(i, map->map[nid]);
    continue;
    }
    - freed += do_shrink_slab(&sc, shrinker, priority);
    + if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
    + sc.nid = 0;
    + ret = do_shrink_slab(&sc, shrinker, priority);
    + if (ret == SHRINK_EMPTY) {
    + clear_bit(i, map->map[nid]);
    + /* pairs with mb in set_shrinker_bit() */
    + smp_mb__after_atomic();
    + ret = do_shrink_slab(&sc, shrinker, priority);
    + if (ret == SHRINK_EMPTY)
    + ret = 0;
    + else
    + set_bit(i, map->map[nid]);
    + }
    + freed += ret;

    if (rwsem_is_contended(&shrinker_rwsem)) {
    freed = freed ? : 1;
    diff --git a/mm/workingset.c b/mm/workingset.c
    index 62c9eb000c4f..dc05574f0a07 100644
    --- a/mm/workingset.c
    +++ b/mm/workingset.c
    @@ -402,6 +402,9 @@ static unsigned long count_shadow_nodes(struct shrinker *shrinker,
    }
    max_nodes = cache >> (RADIX_TREE_MAP_SHIFT - 3);

    + if (!nodes)
    + return SHRINK_EMPTY;
    +
    if (nodes <= max_nodes)
    return 0;
    return nodes - max_nodes;
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