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

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Date:	Wed, 5 Aug 2009 11:40:04 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Minchan Kim <minchan.kim@...il.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

On Wed,  5 Aug 2009 11:29:34 +0900 (JST)
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:

> 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.
> 

Thanks for good explanation. :)

It resulted from patch of David which moved task_struct->oom_ajd 
to mm_struct. I understood it. 

It meant oom_adj was not per-process.

AFAIU, you want to make oom_adj per-process, again. 
And you selected the place with task->singal as per-process.

What I have a question is that why do you select task_struct->signal 
rather than task_struct like old?

What's benefit of using task_struct->signal ?


-- 
Kind regards,
Minchan Kim
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