linux-kernel - Re: [PATCH 1/4] oom: move oom_adj to signal

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Message-Id: <20090805155500.b4692b27.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 5 Aug 2009 15:55:00 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Paul Menage <menage@...gle.com>,
	David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH 1/4] oom: move oom_adj to signal_struct

On Wed, 5 Aug 2009 15:37:01 +0900
Minchan Kim <minchan.kim@...il.com> wrote:
> Hmm. I can't understand why it is troublesome. 
> I think it's related to moving oom_adj to singal_struct. 
> Unfortunately, I can't understand why we have to put oom_adj 
> in singal_struct?
> 
> That's why I have a question to Kosaki a while ago. 
> I can't understand it still. :-(
> 
> Could you elaborate it ?
> 

Current code is as following
==
  do_each_thread(g,p) {
	......
	p = badness();

	record p of highest badness.
  }
  p = higest badness thread.

  Scan all threads which shares mm_struct of p. and check oom_adj
  
==	
Assume a process which has 20000 threads. And 1 of thread has OOM_DISABLE.

Then, at worst, this scan will needs
	(1+2+3+....+20000) * (20000-1) scan. (when ignoring other processes)
even with your patch.

This means the kernel wastes enough long time that Cluster-Management-Software can
detetct this as livelock, and do reboot/cluster-fail-over.

Fixing livelock is not the last goal. I (we) would like to reduct stall time
to reasonable level. If we move oom_adj to signal_struct or mm_struct, scan-cost
will be only 20000. No retry at all.

And, if we can use for_each_process() rather than do_each_thread(),
scan-cost will be 1.

(BTW, "signal" struct is bad name I think, it should be "process" struct ;)


Thanks,
-Kame

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