linux-kernel - Re: [RFC 1/3] oom, sysrq: Skip over oom victims and killed tasks

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Date:	Wed, 13 Jan 2016 10:30:46 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	linux-mm@...ck.org,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 1/3] oom, sysrq: Skip over oom victims and killed tasks

On Tue 12-01-16 16:41:50, David Rientjes wrote:
> On Tue, 12 Jan 2016, Michal Hocko wrote:
> 
> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > index abefeeb42504..2b9dc5129a89 100644
> > --- a/mm/oom_kill.c
> > +++ b/mm/oom_kill.c
> > @@ -326,6 +326,17 @@ static struct task_struct *select_bad_process(struct oom_control *oc,
> >  		case OOM_SCAN_OK:
> >  			break;
> >  		};
> > +
> > +		/*
> > +		 * If we are doing sysrq+f then it doesn't make any sense to
> > +		 * check OOM victim or killed task because it might be stuck
> > +		 * and unable to terminate while the forced OOM might be the
> > +		 * only option left to get the system back to work.
> > +		 */
> > +		if (is_sysrq_oom(oc) && (test_tsk_thread_flag(p, TIF_MEMDIE) ||
> > +				fatal_signal_pending(p)))
> > +			continue;
> > +
> >  		points = oom_badness(p, NULL, oc->nodemask, totalpages);
> >  		if (!points || points < chosen_points)
> >  			continue;
> 
> I think you can make a case for testing TIF_MEMDIE here since there is no 
> chance of a panic from the sysrq trigger.  However, I'm not convinced that 
> checking fatal_signal_pending() is appropriate. 

My thinking was that such a process would get TIF_MEMDIE if it hits the
OOM from the allocator.

> I think it would be 
> better for sysrq+f to first select a process with fatal_signal_pending() 
> set so it silently gets access to memory reserves and then a second 
> sysrq+f to choose a different process, if necessary, because of 
> TIF_MEMDIE.

The disadvantage of this approach is that sysrq+f might silently be
ignored and the administrator doesn't have any signal about that. IMHO
sysrq+f would be much better defined if it _always_ selected and killed
a task. After all it is an explicit administrator action.
-- 
Michal Hocko
SUSE Labs