Date: Wed, 15 Jun 2011 09:12:45 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
To: Michal Hocko <mhocko@suse.cz>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "nishimura@mxp.nes.nec.co.jp" <nishimura@mxp.nes.nec.co.jp>,
        "bsingharora@gmail.com" <bsingharora@gmail.com>,
        "hannes@cmpxchg.org" <hannes@cmpxchg.org>,
        Ying Han <yinghan@google.com>
Subject: Re: [BUGFIX][PATCH 5/5] memcg: fix percpu cached charge draining
 frequency
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	<20110614073651.GA21197@tiehlicka.suse.cz>
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On Tue, 14 Jun 2011 09:36:51 +0200
Michal Hocko <mhocko@suse.cz> wrote:

> On Mon 13-06-11 12:16:48, KAMEZAWA Hiroyuki wrote:
> > From 18b12e53f1cdf6d7feed1f9226c189c34866338c Mon Sep 17 00:00:00 2001
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > Date: Mon, 13 Jun 2011 11:25:43 +0900
> > Subject: [PATCH 5/5] memcg: fix percpu cached charge draining frequency
> > 
> >  For performance, memory cgroup caches some "charge" from res_counter
> >  into per cpu cache. This works well but because it's cache,
> >  it needs to be flushed in some cases. Typical cases are
> >          1. when someone hit limit.
> >          2. when rmdir() is called and need to charges to be 0.
> > 
> > But "1" has problem.
> > 
> > Recently, with large SMP machines, we see many kworker runs because
> > of flushing memcg's cache. Bad things in implementation are
> > that even if a cpu contains a cache for memcg not related to
> > a memcg which hits limit, drain code is called.
> > 
> > This patch does
> > 	D) don't call at softlimit reclaim.
> 
> I think this needs some justification. The decision is not that
> obvious IMO. I would say that this is a good decision because cached
> charges will not help to free any memory (at least not directly) during
> background reclaim. What about something like:
> "
> We are not draining per cpu cached charges during soft limit reclaim 
> because background reclaim doesn't care about charges. It tries to free
> some memory and charges will not give any.
> Cached charges might influence only selection of the biggest soft limit
> offender but as the call is done only after the selection has been
> already done it makes no change.
> "
> 
> Anyway, wouldn't it be better to have this change separate from the
> async draining logic change?

Hmm. I think calling "draining" at softlimit is just a bug.

Thanks,
-Kame


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