linux-kernel - Re: [PATCH -mm 0/8] memcg: reparent kmem on css offline

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Message-ID: <20140708221906.GC29639@cmpxchg.org>
Date:	Tue, 8 Jul 2014 18:19:06 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	akpm@...ux-foundation.org, mhocko@...e.cz, cl@...ux.com,
	glommer@...il.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm 0/8] memcg: reparent kmem on css offline

On Mon, Jul 07, 2014 at 09:14:15PM +0400, Vladimir Davydov wrote:
> 07.07.2014 18:25, Johannes Weiner:
> >In addition, Tejun made offlined css iterable and split css_tryget()
> >and css_tryget_online(), which would allow memcg to pin the css until
> >the last charge is gone while continuing to iterate and reclaim it on
> >hierarchical pressure, even after it was offlined.
> 
> One more question.
> 
> With reparenting enabled, the number of cgroups (lruvecs) that must be
> iterated on global reclaim is bound by the number of live containers,
> while w/o reparenting it's practically unbound, isn't it? Won't it be
> the source of latency spikes?

It might deteriorate a little bit, but it is a self-correcting problem
as soon as memory pressure kicks.  Creating and destroying cgroups is
serialized at a global level, so I would expect the cost of doing that
at a high rate to become a problem before the csss become an issue for
the reclaim scanner.

At some point we will probably have to make the global reclaim cgroup
walk in shrink_zone() intermittent, but I'm not aware of any problems
with it so far.
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