[PATCH 9/9] slab: remove slub sysfs interface files early for empty memcg caches

[Tejun Heo]

With kmem cgroup support enabled, kmem_caches can be created and
destroyed frequently and a great number of near empty kmem_caches can
accumulate if there are a lot of transient cgroups and the system is
not under memory pressure.  When memory reclaim starts under such
conditions, it can lead to consecutive deactivation and destruction of
many kmem_caches, easily hundreds of thousands on moderately large
systems, exposing scalability issues in the current slab management
code.  This is one of the patches to address the issue.

Each cache has a number of sysfs interface files under
/sys/kernel/slab.  On a system with a lot of memory and transient
memcgs, the number of interface files which have to be removed once
memory reclaim kicks in can reach millions.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-by: Jay Vana <jsvana@xxxxxx>
Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
 mm/slub.c | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 8621940..41a3da7 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3951,8 +3951,20 @@ int __kmem_cache_shrink(struct kmem_cache *s)
 #ifdef CONFIG_MEMCG
 static void kmemcg_cache_deact_after_rcu(struct kmem_cache *s)
 {
-	/* called with all the locks held after a sched RCU grace period */
-	__kmem_cache_shrink(s);
+	/*
+	 * Called with all the locks held after a sched RCU grace period.
+	 * Even if @s becomes empty after shrinking, we can't know that @s
+	 * doesn't have allocations already in-flight and thus can't
+	 * destroy @s until the associated memcg is released.
+	 *
+	 * However, let's remove the sysfs files for empty caches here.
+	 * Each cache has a lot of interface files which aren't
+	 * particularly useful for empty draining caches; otherwise, we can
+	 * easily end up with millions of unnecessary sysfs files on
+	 * systems which have a lot of memory and transient cgroups.
+	 */
+	if (!__kmem_cache_shrink(s))
+		sysfs_slab_remove(s);
 }
 
 void __kmemcg_cache_deactivate(struct kmem_cache *s)
@@ -5650,6 +5662,15 @@ static void sysfs_slab_remove(struct kmem_cache *s)
 		 */
 		return;
 
+	if (!s->kobj.state_in_sysfs)
+		/*
+		 * For a memcg cache, this may be called during
+		 * deactivation and again on shutdown.  Remove only once.
+		 * A cache is never shut down before deactivation is
+		 * complete, so no need to worry about synchronization.
+		 */
+		return;
+
 #ifdef CONFIG_MEMCG
 	kset_unregister(s->memcg_kset);
 #endif
-- 
2.9.3