linux-kernel - Re: [PATCH] oom killer: break from infinite loop

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Message-ID: <20100331063007.GN3308@balbir.in.ibm.com>
Date:	Wed, 31 Mar 2010 12:00:07 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	David Rientjes <rientjes@...gle.com>, anfei <anfei.zhou@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	nishimura@....nes.nec.co.jp, Mel Gorman <mel@....ul.ie>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] oom killer: break from infinite loop

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2010-03-31 15:13:56]:

> On Tue, 30 Mar 2010 23:07:08 -0700 (PDT)
> David Rientjes <rientjes@...gle.com> wrote:
> 
> > On Wed, 31 Mar 2010, KAMEZAWA Hiroyuki wrote:
> > 
> > > > > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > > > > index 0cb1ca4..9e89a29 100644
> > > > > --- a/mm/oom_kill.c
> > > > > +++ b/mm/oom_kill.c
> > > > > @@ -510,8 +510,10 @@ retry:
> > > > >  	if (PTR_ERR(p) == -1UL)
> > > > >  		goto out;
> > > > >  
> > > > > -	if (!p)
> > > > > -		p = current;
> > > > > +	if (!p) {
> > > > > +		read_unlock(&tasklist_lock);
> > > > > +		panic("Out of memory and no killable processes...\n");
> > > > > +	}
> > > > >  
> > > > >  	if (oom_kill_process(p, gfp_mask, 0, points, limit, mem,
> > > > >  				"Memory cgroup out of memory"))
> > > > > 
> > > > 
> > > > This actually does appear to be necessary but for a different reason: if 
> > > > current is unkillable because it has OOM_DISABLE, for example, then 
> > > > oom_kill_process() will repeatedly fail and mem_cgroup_out_of_memory() 
> > > > will infinitely loop.
> > > > 
> > > > Kame-san?
> > > > 
> > > 
> > > When a memcg goes into OOM and it only has unkillable processes (OOM_DISABLE),
> > > we can do nothing. (we can't panic because container's death != system death.)
> > > 
> > > Because memcg itself has mutex+waitqueue for mutual execusion of OOM killer, 
> > > I think infinite-loop will not be critical probelm for the whole system.
> > > 
> > > And, now, memcg has oom-kill-disable + oom-kill-notifier features.
> > > So, If a memcg goes into OOM and there is no killable process, but oom-kill is
> > > not disabled by memcg.....it means system admin's mis-configuraton.
> > > 
> > > He can stop inifite loop by hand, anyway.
> > > # echo 1 > ..../group_A/memory.oom_control
> > > 
> > 
> > Then we should be able to do this since current is by definition 
> > unkillable since it was not found in select_bad_process(), right?
> 
> To me, this patch is acceptable and seems reasnoable.
> 
> But I didn't joined to memcg development when this check was added
> and don't know why kill current..
>

The reason for adding current was that we did not want to loop
forever, since it stops forward progress - no error/no forward
progress. It made sense to oom kill the current process, so that the
cgroup admin could look at what went wrong.
 
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=c7ba5c9e8176704bfac0729875fa62798037584d
> 
> Addinc Balbir to CC. Maybe situation is changed now.
> Because we can stop inifinite loop (by hand) and there is no rushing oom-kill
> callers, this change is acceptable.
>

By hand is not always possible if we have a large number of cgroups
(I've seen a setup with 2000 cgroups on libcgroup ML). 2000 cgroups *
number of processes make the situation complex. I think using OOM
notifier is now another way of handling such a situation.
 
-- 
	Three Cheers,
	Balbir
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