linux-kernel - Re: [patch -mm v2] mm: introduce oom_adj

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Message-Id: <20090803212945.CC2F.A69D9226@jp.fujitsu.com>
Date:	Mon,  3 Aug 2009 21:32:40 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Paul Menage <menage@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [patch -mm v2] mm: introduce oom_adj_child

Hi

Sorry for queue jumping. I have one question.


> > >  - /proc/pid/oom_score is inconsistent when the thread that set the
> > >    effective per-mm oom_adj exits and it is now obsolete since you have
> > >    no way to determine what the next effective oom_adj value shall be.
> > > 
> > plz re-caluculate it. it's not a big job if done in lazy way.
> > 
> 
> You can't recalculate it if all the remaining threads have a different 
> oom_adj value than the effective oom_adj value from the thread that is now 
> exited.  There is no assumption that, for instance, the most negative 
> oom_adj value shall then be used.  Imagine the effective oom_adj value 
> being +15 and a thread sharing the same memory has an oom_adj value of 
> -16.  Under no reasonable circumstance should the oom preference of the 
> entire thread then change to -16 just because its the side-effect of a 
> thread exiting.

Why do we need recaluculate AT thread exiting time?
it is only used when oom_score is readed or actual OOM happend.
both those are slow-path.


> 
> That's the _entire_ reason why we need consistency in oom_adj values so 
> that userspace is aware of how the oom killer really works and chooses 
> tasks.  I understand that it differs from the previously allowed behavior, 
> but those userspace applications need to be fixed if, for no other reason, 
> they are now consistent with how the oom killer kills tasks.  I think 
> that's a very worthwhile goal and the cost of moving to a new interface 
> such as /proc/pid/oom_adj_child to have the same inheritance property that 
> was available in the past is justified.



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