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.

While a memcg kmem_cache is listed on its root cache's ->children list,
there is no direct way to iterate all kmem_caches which are assocaited
with a memory cgroup.  The only way to iterate them is walking all
caches while filtering out caches which don't match, which would be most
of them.

This makes memcg destruction operations O(N^2) where N is the total
number of slab caches which can be huge.  This combined with the
synchronous RCU operations can tie up a CPU and affect the whole machine
for many hours when memory reclaim triggers offlining and destruction of
the stale memcgs.

This patch adds mem_cgroup->kmem_caches list which goes through
memcg_cache_params->kmem_caches_node of all kmem_caches which are
associated with the memcg.  All memcg specific iterations, including
stat file access, are updated to use the new list instead.

Link: http://lkml.kernel.org/r/20170117235411.9408-6-tj@kernel.orgSigned-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>