linux-kernel - Re: [PATCH 8/9] slab: remove synchronous synchronize_sched() from memcg cache deactivation path

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Message-ID: <20170114135727.GG2668@esperanza>
Date:   Sat, 14 Jan 2017 16:57:27 +0300
From:   Vladimir Davydov <vdavydov@...antool.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org, jsvana@...com,
        hannes@...xchg.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, cgroups@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH 8/9] slab: remove synchronous synchronize_sched() from
 memcg cache deactivation path

On Sat, Jan 14, 2017 at 12:54:48AM -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.
> 
> 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.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Reported-by: Jay Vana <jsvana@...com>
> Cc: Vladimir Davydov <vdavydov.dev@...il.com>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Pekka Enberg <penberg@...nel.org>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>

I don't think there's much point in having the infrastructure for this
in slab_common.c, as only SLUB needs it, but it isn't a show stopper.

Acked-by: Vladimir Davydov <vdavydov.dev@...il.com>