linux-kernel - Re: [patch 1/2] mm, memcg: avoid oom notification when current needs access to memory reserves

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Message-ID: <alpine.DEB.2.02.1312111434200.7354@chino.kir.corp.google.com>
Date:	Wed, 11 Dec 2013 14:40:24 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Michal Hocko <mhocko@...e.cz>
cc:	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	cgroups@...r.kernel.org
Subject: Re: [patch 1/2] mm, memcg: avoid oom notification when current needs
 access to memory reserves

On Wed, 11 Dec 2013, Michal Hocko wrote:

> > Triggering a pointless notification with PF_EXITING is rare, yet one 
> > pointless notification can be avoided with the patch. 
> 
> Sigh. Yes it will avoid one particular and rare race. There will still
> be notifications without oom kills.
> 

Would you prefer doing the mem_cgroup_oom_notify() in two places instead:

 - immediately before doing oom_kill_process() when it's guaranteed that
   the kernel would have killed something, and

 - when memory.oom_control == 1 in mem_cgroup_oom_synchronize()?

> Anyway.
> Does the reclaim make any sense for PF_EXITING tasks? Shouldn't we
> simply bypass charges of these tasks automatically. Those tasks will
> free some memory anyway so why to trigger reclaim and potentially OOM
> in the first place? Do we need to go via TIF_MEMDIE loop in the first
> place?
> 

I don't see any reason to make an optimization there since they will get 
TIF_MEMDIE set if reclaim has failed on one of their charges or if it 
results in a system oom through the page allocator's oom killer.  It would 
be nice to ensure reclaim has had a chance to free memory in the presence 
of any other potential parallel memory freeing.
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