linux-kernel - Re: [PATCH] mm: slub: replace kick_all_cpus_sync with synchronize_sched in kmem_cache

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Message-ID: <20160401154803.GL3448@twins.programming.kicks-ass.net>
Date:	Fri, 1 Apr 2016 17:48:03 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vladimir Davydov <vdavydov@...tuozzo.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: slub: replace kick_all_cpus_sync with
 synchronize_sched in kmem_cache_shrink

On Fri, Apr 01, 2016 at 03:30:17PM +0300, Vladimir Davydov wrote:
> When we call __kmem_cache_shrink on memory cgroup removal, we need to
> synchronize kmem_cache->cpu_partial update with put_cpu_partial that
> might be running on other cpus. Currently, we achieve that by using
> kick_all_cpus_sync, which works as a system wide memory barrier. Though
> fast it is, this method has a flow - it issues a lot of IPIs, which
> might hurt high performance or real-time workloads.
> 
> To fix this, let's replace kick_all_cpus_sync with synchronize_sched.
> Although the latter one may take much longer to finish, it shouldn't be
> a problem in this particular case, because memory cgroups are destroyed
> asynchronously from a workqueue so that no user visible effects should
> be introduced. OTOH, it will save us from excessive IPIs when someone
> removes a cgroup.
> 
> Anyway, even if using synchronize_sched turns out to take too long, we
> can always introduce a kind of __kmem_cache_shrink batching so that this
> method would only be called once per one cgroup destruction (not per
> each per memcg kmem cache as it is now).
> 
> Reported-and-suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Vladimir Davydov <vdavydov@...tuozzo.com>

Thanks!

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>