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Date: Wed, 25 Nov 2009 08:59:55 +0530
From: Balbir Singh <balbir@linux.vnet.ibm.com>
To: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Andrew Morton <akpm@linux-foundation.org>,
       LKML <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>,
       stable <stable@kernel.org>, David Rientjes <rientjes@google.com>,
       KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Subject: Re: [BUGFIX][PATCH -mmotm] memcg: avoid oom-killing innocent task
	in case of use_hierarchy
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* nishimura@mxp.nes.nec.co.jp <nishimura@mxp.nes.nec.co.jp> [2009-11-25 08:49:10]:

> On Tue, 24 Nov 2009 22:34:02 +0530, Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> > * Daisuke Nishimura <d-nishimura@mtf.biglobe.ne.jp> [2009-11-24 23:00:29]:
> > 
> > > On Tue, 24 Nov 2009 19:01:54 +0530
> > > Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> > > 
> > > > On Tue, Nov 24, 2009 at 11:27 AM, Daisuke Nishimura
> > > > <nishimura@mxp.nes.nec.co.jp> wrote:
> > > > > task_in_mem_cgroup(), which is called by select_bad_process() to check whether
> > > > > a task can be a candidate for being oom-killed from memcg's limit, checks
> > > > > "curr->use_hierarchy"("curr" is the mem_cgroup the task belongs to).
> > > > >
> > > > > But this check return true(it's false positive) when:
> > > > >
> > > > > ? ? ? ?<some path>/00 ? ? ? ? ?use_hierarchy == 0 ? ? ?<- hitting limit
> > > > > ? ? ? ? ?<some path>/00/aa ? ? use_hierarchy == 1 ? ? ?<- "curr"
> > > > >
> > > > > This leads to killing an innocent task in 00/aa. This patch is a fix for this
> > > > > bug. And this patch also fixes the arg for mem_cgroup_print_oom_info(). We
> > > > > should print information of mem_cgroup which the task being killed, not current,
> > > > > belongs to.
> > > > >
> > > > 
> > > > Quick Question: What happens if <some path>/00 has no tasks in it
> > > > after your patches?
> > > > 
> > > Nothing would happen because <some path>/00 never hit its limit.
> > 
> > Why not? I am talking of a scenario where <some path>/00 is set to a
> > limit (similar to your example) and hits its limit, but the groups
> > under it have no limits, but tasks. Shouldn't we be scanning
> > <some path>/00/aa as well?
> > 
> > > 
> > > The bug that this patch fixes is:
> > > 
> > > - create a dir <some path>/00 and set some limits.
> > > - create a sub dir <some path>/00/aa w/o any limits, and enable hierarchy.
> > > - run some programs in both in 00 and 00/aa. programs in 00 should be
> > >   big enough to cause oom by its limit.
> > > - when oom happens by 00's limit, tasks in 00/aa can also be killed.
> > >
> > 
> > To be honest, the last part is fair, specifically if 00/aa has a task
> > that is really the heaviest task as per the oom logic. no? Are you
> > suggesting that only tasks in <some path>/00 should be selected by the
> > oom logic? 
> > 
> All of your comments would be rational if hierarchy is enabled in 00(it's
> also enabled in 00/aa automatically in this case).
> I'm saying about the case where it's disabled in 00 but enabled in 00/aa.
> 

OK, I misunderstood the example then, so even though hierarchy is
disabled, we kill a task in 00/aa when 00 hits the limit. Thanks for
clarifying.

> In this scenario, charges by tasks in 00/aa is(and should not be) charged to 00.
> And oom caused by 00's limit should not affect the task in 00/aa.
> 
> 
> Regards,
> Daisuke Nishimura.
> 
> --
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> 

-- 
	Balbir
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