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Date: Wed, 28 Sep 2016 11:09:53 +0300
From: Vladimir Davydov <vdavydov.dev@gmail.com>
Subject: Re: [Bug 172981] New: [bisected] SLAB: extreme load averages and
 over 2000 kworker threads
Message-ID: <20160928080953.GA20312@esperanza>
References: <bug-172981-27@https.bugzilla.kernel.org/>
 <20160927111059.282a35c89266202d3cb2f953@linux-foundation.org>
 <20160928020347.GA21129@cmpxchg.org>
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To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andrew Morton <akpm@linux-foundation.org>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, bugzilla-daemon@bugzilla.kernel.org, dsmythies@telus.net, linux-mm@kvack.org

On Tue, Sep 27, 2016 at 10:03:47PM -0400, Johannes Weiner wrote:
> [CC Vladimir]
> 
> These are the delayed memcg cache allocations, where in a fresh memcg
> that doesn't have per-memcg caches yet, every accounted allocation
> schedules a kmalloc work item in __memcg_schedule_kmem_cache_create()
> until the cache is finally available. It looks like those can be many
> more than the number of slab caches in existence, if there is a storm
> of slab allocations before the workers get a chance to run.
> 
> Vladimir, what do you think of embedding the work item into the
> memcg_cache_array? That way we make sure we have exactly one work per
> cache and not an unbounded number of them. The downside of course is
> that we'd have to keep these things around as long as the memcg is in
> existence, but that's the only place I can think of that allows us to
> serialize this.

We could set the entry of the root_cache->memcg_params.memcg_caches
array corresponding to the cache being created to a special value, say
(void*)1, and skip scheduling cache creation work on kmalloc if the
caller sees it. I'm not sure it's really worth it though, because
work_struct isn't that big (at least, in comparison with the cache
itself) to avoid embedding it at all costs.

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