slab: implement slab_root_caches list

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_caches currently lists all caches including root and memcg ones.
This is the only data structure which lists the root caches and
iterating root caches can only be done by walking the list while
skipping over memcg caches.  As there can be a huge number of memcg
caches, this can become very expensive.

This also can make /proc/slabinfo behave very badly.  seq_file processes
reads in 4k chunks and seeks to the previous Nth position on slab_caches
list to resume after each chunk.  With a lot of memcg cache churns on
the list, reading /proc/slabinfo can become very slow and its content
often ends up with duplicate and/or missing entries.

This patch adds a new list slab_root_caches which lists only the root
caches.  When memcg is not enabled, it becomes just an alias of
slab_caches.  memcg specific list operations are collected into
memcg_[un]link_cache().

Link: http://lkml.kernel.org/r/20170117235411.9408-7-tj@kernel.org


Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Jay Vana <jsvana@fb.com>
Acked-by: Vladimir Davydov <vdavydov@tarantool.org>
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>