linux-kernel - Re: [PATCH 4/5] oom: don't kill random process

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Date:	Mon, 23 May 2011 18:39:49 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, caiqian@...hat.com, hughd@...gle.com,
	kamezawa.hiroyu@...fujitsu.com, minchan.kim@...il.com,
	oleg@...hat.com
Subject: Re: [PATCH 4/5] oom: don't kill random process

On Tue, 24 May 2011, KOSAKI Motohiro wrote:

> > > Also, this patch move finding sacrifice child logic into
> > > select_bad_process(). It's necessary to implement adequate
> > > no root bonus recalculation. and it makes good side effect,
> > > current logic doesn't behave as the doc.
> > > 
> > 
> > This is unnecessary and just makes the oom killer egregiously long.  We
> > are already diagnosing problems here at Google where the oom killer holds
> > tasklist_lock on the readside for far too long, causing other cpus waiting
> > for a write_lock_irq(&tasklist_lock) to encounter issues when irqs are
> > disabled and it is spinning.  A second tasklist scan is simply a
> > non-starter.
> > 
> >   [ This is also one of the reasons why we needed to introduce
> >     mm->oom_disable_count to prevent a second, expensive tasklist scan. ]
> 
> You misunderstand the code. Both select_bad_process() and oom_kill_process()
> are under tasklist_lock(). IOW, no change lock holding time.
> 

A second iteration through the tasklist in select_bad_process() will 
extend the time that tasklist_lock is held, which is what your patch does.  
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