linux-kernel - Re: [BUGFIX][PATCH 5/5] memcg: fix percpu cached charge draining frequency

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Date:	Wed, 15 Jun 2011 09:15:00 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"bsingharora@...il.com" <bsingharora@...il.com>,
	"hannes@...xchg.org" <hannes@...xchg.org>,
	Ying Han <yinghan@...gle.com>
Subject: Re: [BUGFIX][PATCH 5/5] memcg: fix percpu cached charge draining
 frequency

On Wed 15-06-11 10:12:02, KAMEZAWA Hiroyuki wrote:
> On Wed, 15 Jun 2011 09:12:45 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> 
> > On Tue, 14 Jun 2011 09:36:51 +0200
> > Michal Hocko <mhocko@...e.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@...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.
> > 
> I'll divide patches.

Thanks.

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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