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

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Date:	Wed,  5 Aug 2009 11:29:34 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	LKML <linux-kernel@...r.kernel.org>,
	Paul Menage <menage@...gle.com>,
	David Rientjes <rientjes@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.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

Hi

> Hi, Kosaki. 
> 
> I am so late to invole this thread. 
> But let me have a question. 
> 
> What's advantage of placing oom_adj in singal rather than task ?
> I mean task->oom_adj and task->signal->oom_adj ?
> 
> I am sorry if you already discussed it at last threads. 

Not sorry. that's very good question.

I'm trying to explain the detailed intention of commit 2ff05b2b4eac
(move oom_adj to mm_struct).

In 2.6.30, OOM logic callflow is here.

__out_of_memory
	select_bad_process		for each task
		badness			calculate badness of one task
	oom_kill_process		search child
		oom_kill_task		kill target task and mm shared tasks with it

example, process-A have two thread, thread-A and thread-B and it 
have very fat memory.
And, each thread have following likes oom property.

	thread-A: oom_adj = OOM_DISABLE, oom_score = 0
	thread-B: oom_adj = 0,           oom_score = very-high

Then, select_bad_process() select thread-B, but oom_kill_task refuse
kill the task because thread-A have OOM_DISABLE.
__out_of_memory() call select_bad_process() again. but select_bad_process()
select the same task. It mean kernel fall in the livelock.

The fact is, select_bad_process() must select killable task. otherwise
OOM logic go into livelock.

Is this enough explanation? thanks.

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