linux-kernel - Re: [PATCH 1/3 for 2.6.38] oom: oom_kill_process: don't set TIF

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Date:	Tue, 15 Mar 2011 12:51:28 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Oleg Nesterov <oleg@...hat.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrey Vagin <avagin@...nvz.org>,
	Frantisek Hrbata <fhrbata@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3 for 2.6.38] oom: oom_kill_process: don't set TIF_MEMDIE
 if !p->mm

On Tue, 15 Mar 2011, Oleg Nesterov wrote:

> When I did this change I looked at 81236810226f71bd9ff77321c8e8276dae7efc61
> and the changelog says:
> 
> 	__oom_kill_task() is called to elevate the task's timeslice and give it
> 	access to memory reserves so that it may quickly exit.
> 
> 	This privilege is unnecessary, however, if the task has already detached
> 	its mm.
> 
> Now you are saing this is pointless.
> 

If you have the commit id, do a "git blame 8123681022", because I see a:

5081dde3 (Nick Piggin        2006-09-25 23:31:32 -0700 222)             if (!p->mm)
5081dde3 (Nick Piggin        2006-09-25 23:31:32 -0700 223)                     continue;

in select_bad_process() and it's also iterating over every thread:

a49335cc (Paul Jackson       2005-09-06 15:18:09 -0700 215)     do_each_thread(g, p) {

It's pointless since oom-skip-zombies-when-iterating-tasklist.patch in 
-mm reintroduced the filter for !p->mm in select_bad_process() which was 
still there when 81236810 was merged; it's a small optimization, though, 
to avoid races where the mm becomes detached between the process' 
selection in select_bad_process() and its kill in oom_kill_process().

> The problem is, we can't trust per-thread PF_EXITING checks. But I guess
> we will discuss this more anyway.
> 

My approach, as you saw with 
oom-avoid-deferring-oom-killer-if-exiting-task-is-being-traced.patch in 
-mm is to add exceptions to the oom killer when we can't trust that 
PF_EXITING will soon be exiting.  I think that's a much more long-term 
maintainable solution instead of inferring the status of a thread based on 
external circumstances (such as number of threads in the thread group) 
that could easily change out from under us and once again break the oom 
killer.
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