linux-kernel - Re: [RFC] oom-kill: give the dying task a higher priority

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Message-Id: <20100603083259.7231.A69D9226@jp.fujitsu.com>
Date:	Thu,  3 Jun 2010 08:36:39 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Minchan Kim <minchan.kim@...il.com>, balbir@...ux.vnet.ibm.com,
	Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mel@....ul.ie>, williams@...hat.com
Subject: Re: [RFC] oom-kill: give the dying task a higher priority

> On Wed, 2 Jun 2010, KOSAKI Motohiro wrote:
> 
> > > > @@ -291,9 +309,10 @@ static struct task_struct *select_bad_process(unsigned long *ppoints,
> > > >  		 * Otherwise we could get an easy OOM deadlock.
> > > >  		 */
> > > >  		if (p->flags & PF_EXITING) {
> > > > -			if (p != current)
> > > > +			if (p != current) {
> > > > +				boost_dying_task_prio(p, mem);
> > > >  				return ERR_PTR(-1UL);
> > > > -
> > > > +			}
> > > >  			chosen = p;
> > > >  			*ppoints = ULONG_MAX;
> > > >  		}
> > > 
> > > This has the potential to actually make it harder to free memory if p is 
> > > waiting to acquire a writelock on mm->mmap_sem in the exit path while the 
> > > thread holding mm->mmap_sem is trying to run.
> > 
> > if p is waiting, changing prio have no effect. It continue tol wait to release mmap_sem.
> > 
> 
> And that can reduce the runtime of the thread holding a writelock on 
> mm->mmap_sem, making the exit actually take longer than without the patch 
> if its priority is significantly higher, especially on smaller machines.

If p need mmap_sem, p is going to sleep to wait mmap_sem. if p doesn't,
quickly exit is good thing. In other word, task fairness is not our goal
when oom occur.



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