On Tue, Jan 17, 2017 at 03:54:04PM -0800, Tejun Heo wrote:

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.

SLAB_DESTORY_BY_RCU caches need to flush all RCU operations before
destruction because slab pages are freed through RCU and they need to
be able to dereference the associated kmem_cache.  Currently, it's
done synchronously with rcu_barrier().  As rcu_barrier() is expensive
time-wise, slab implements a batching mechanism so that rcu_barrier()
can be done for multiple caches at the same time.

Unfortunately, the rcu_barrier() is in synchronous path which is
called while holding cgroup_mutex and the batching is too limited to
be actually helpful.

This patch updates the cache release path so that the batching is
asynchronous and global.  All SLAB_DESTORY_BY_RCU caches are queued
globally and a work item consumes the list.  The work item calls
rcu_barrier() only once for all caches that are currently queued.

* release_caches() is removed and shutdown_cache() now either directly
  release the cache or schedules a RCU callback to do that.  This
  makes the cache inaccessible once shutdown_cache() is called and
  makes it impossible for shutdown_memcg_caches() to do memcg-specific
  cleanups afterwards.  Move memcg-specific part into a helper,
  unlink_memcg_cache(), and make shutdown_cache() call it directly.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Jay Vana <redacted>
Cc: Vladimir Davydov <redacted>
Cc: Christoph Lameter <redacted>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <redacted>
Cc: Andrew Morton <akpm@linux-foundation.org>

Acked-by: Vladimir Davydov <redacted>

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