linux-kernel - Re: [PATCH] oom: skip frozen tasks

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Message-ID: <4E574CA5.4010701@openvz.org>
Date:	Fri, 26 Aug 2011 11:35:01 +0400
From:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
To:	David Rientjes <rientjes@...gle.com>
CC:	Michal Hocko <mhocko@...e.cz>, Oleg Nesterov <oleg@...hat.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: [PATCH] oom: skip frozen tasks

David Rientjes wrote:
> On Thu, 25 Aug 2011, Michal Hocko wrote:
>
>>>>> That's obviously false since we call oom_killer_disable() in
>>>>> freeze_processes() to disable the oom killer from ever being called in the
>>>>> first place, so this is something you need to resolve with Rafael before
>>>>> you cause more machines to panic.
>>>>
>>>> I didn't mean suspend/resume path (that is protected by oom_killer_disabled)
>>>> so the patch doesn't make any change.
>>>
>>> Confused... freeze_processes() does try_to_freeze_tasks() before
>>> oom_killer_disable() ?
>>
>> Yes you are right, I must have been blind.
>>
>> Now I see the point. We do not want to panic while we are suspending and
>> the memory is really low just because all the userspace is already in
>> the the fridge.
>> Sorry for confusion.
>>
>> I still do not follow the oom_killer_disable note from David, though.
>>
>
> oom_killer_disable() was added to that path for a reason when all threads
> are frozen: memory allocations still occur in the suspend path in an oom
> condition and adding the oom_killer_disable() will cause those
> allocations to fail rather than sending pointless SIGKILLs to frozen
> threads.
>
> Now consider if the only _eligible_ threads for oom kill (because of
> cpusets or mempolicies) are those that are frozen.  We certainly do not
> want to panic because other cpusets are still getting work done.  We'd
> either want to add a mem to the cpuset or thaw the processes because the
> cpuset is oom.
>
> You can't just selectively skip certain threads when their state can be
> temporary without risking a panic.  That's why this patch is a
> non-starter.
>
> A much better solution would be to lower the badness score that the oom
> killer uses for PF_FROZEN threads so that they aren't considered a
> priority for kill unless there's nothing else left to kill.

Anyway, oom killer shouldn't loop endlessly if it see TIF_MEMDIE on frozen task,
it must go on and try to kill somebody else. We cannot wait for thawing this task.
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