slab: remove synchronous synchronize_sched() from memcg cache deactivation path

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.

slub uses synchronize_sched() to deactivate a memcg cache.
synchronize_sched() is an expensive and slow operation and doesn't scale
when a huge number of caches are destroyed back-to-back.  While there
used to be a simple batching mechanism, the batching was too restricted
to be helpful.

This patch implements slab_deactivate_memcg_cache_rcu_sched() which slub
can use to schedule sched RCU callback instead of performing
synchronize_sched() synchronously while holding cgroup_mutex.  While
this adds online cpus, mems and slab_mutex operations, operating on
these locks back-to-back from the same kworker, which is what's gonna
happen when there are many to deactivate, isn't expensive at all and
this gets rid of the scalability problem completely.

Link: http://lkml.kernel.org/r/20170117235411.9408-9-tj@kernel.org
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Jay Vana <jsvana@fb.com>
Acked-by: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>