Re: [PATCH 6/9] slab: don't put memcg caches on slab_caches list

[Vladimir Davydov]

On Sat, Jan 14, 2017 at 12:54:46AM -0500, 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_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.
> 
> As the previous patch made it unnecessary to walk slab_caches to
> iterate memcg-specific caches, there is no reason to keep memcg caches
> on the list.  This patch makes slab_caches include only the root
> caches.  As this makes slab_cache->list unused for memcg caches,
> ->memcg_params.children_node is removed and ->list is used instead.
> 
> 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>
> ---
>  include/linux/slab.h |  3 ---
>  mm/slab.h            |  3 +--
>  mm/slab_common.c     | 58 +++++++++++++++++++++++++---------------------------
>  3 files changed, 29 insertions(+), 35 deletions(-)

IIRC the slab_caches list is also used on cpu/mem online/offline, so you
have to patch those places to ensure that memcg caches get updated too.
Other than that the patch looks good to me.