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

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Date:	Mon, 23 May 2011 18:58:26 -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:

> > > > 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.
> 
> It never happen usual case. Plz think when happen all process score = 1.
> 

I don't care if it happens in the usual case or extremely rare case.  It 
significantly increases the amount of time that tasklist_lock is held 
which causes writelock starvation on other cpus and causes issues, 
especially if the cpu being starved is updating the timer because it has 
irqs disabled, i.e. write_lock_irq(&tasklist_lock) usually in the clone or 
exit path.  We can do better than that, and that's why I proposed my patch 
to CAI that increases the resolution of the scoring and makes the root 
process bonus proportional to the amount of used memory.
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