linux-kernel - Re: [BUGFIX][PATCH] memcg: fix behavior of per cpu charge cache draining.

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Date:	Wed, 8 Jun 2011 11:15:11 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Ying Han <yinghan@...gle.com>,
	"hannes@...xchg.org" <hannes@...xchg.org>,
	Balbir Singh <bsingharora@...il.com>
Subject: Re: [BUGFIX][PATCH] memcg: fix behavior of per cpu charge cache
 draining.

On Wed 08-06-11 15:29:01, KAMEZAWA Hiroyuki wrote:
> On Wed, 8 Jun 2011 14:49:34 +0900
> Daisuke Nishimura <nishimura@....nes.nec.co.jp> wrote:
> 
> > I have a few minor comments.
> > 
> > On Wed, 8 Jun 2011 14:05:18 +0900
> > KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> > 
> > > This patch is made against mainline git tree.
> > > ==
> > > From d1372da4d3c6f8051b5b1cf7b5e8b45a8094b388 Mon Sep 17 00:00:00 2001
> > > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > > Date: Wed, 8 Jun 2011 13:51:11 +0900
> > > Subject: [BUGFIX][PATCH] memcg: fix behavior of per cpu charge cache draining.
> > > 
> > > 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/%d:%d when
> > > memcg hit limit. It is because of flushing memcg's percpu cache. 
> > > Bad things in implementation are
> > > 
> > > a) it's called before calling try_to_free_mem_cgroup_pages()
> > >    so, it's called immidiately when a task hit limit.
> > >    (I thought it was better to avoid to run into memory reclaim.
> > >     But it was wrong decision.)
> > > 
> > > b) Even if a cpu contains a cache for memcg not related to
> > >    a memcg which hits limit, drain code is called.
> > > 
> > > This patch fixes a) and b) by
> > > 
> > > A) delay calling of flushing until one run of try_to_free...
> > >    Then, the number of calling is much decreased.
> > > B) check percpu cache contains a useful data or not.
> > > plus
> > > C) check asynchronous percpu draining doesn't run on the cpu.
> > > 
> > > Reported-by: Ying Han <yinghan@...gle.com>
> > > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

Looks good to me.
Reviewed-by: Michal Hocko <mhocko@...e.cz>

One minor note though. 
AFAICS we can end up having CHARGE_BATCH * (NR_ONLINE_CPU) pages pre-charged
for a group which would be freed by drain_all_stock_async so we could get
under the limit and so we could omit direct reclaim, or?

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