linux-kernel - Re: [patch] mm: oom_kill: revert 3% system memory bonus for privileged tasks

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Message-ID: <alpine.DEB.2.02.1401151614480.15665@chino.kir.corp.google.com>
Date:	Wed, 15 Jan 2014 16:18:47 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Johannes Weiner <hannes@...xchg.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] mm: oom_kill: revert 3% system memory bonus for privileged
 tasks

On Wed, 15 Jan 2014, Johannes Weiner wrote:

> With a63d83f427fb ("oom: badness heuristic rewrite"), the OOM killer
> tries to avoid killing privileged tasks by subtracting 3% of overall
> memory (system or cgroup) from their per-task consumption.  But as a
> result, all root tasks that consume less than 3% of overall memory are
> considered equal, and so it only takes 33+ privileged tasks pushing
> the system out of memory for the OOM killer to do something stupid and
> kill sshd or dhclient.  For example, on a 32G machine it can't tell
> the difference between the 1M agetty and the 10G fork bomb member.
> 
> The changelog describes this 3% boost as the equivalent to the global
> overcommit limit being 3% higher for privileged tasks, but this is not
> the same as discounting 3% of overall memory from _every privileged
> task individually_ during OOM selection.
> 
> Revert back to the old priority boost of pretending root tasks are
> only a quarter of their actual size.
> 

Unfortunately, I think this could potentially be too much of a bonus.  On 
your same 32GB machine, if a root process is using 18GB and a user process 
is using 14GB, the user process ends up getting selected while the current 
discount of 3% still selects the root process.

I do like the idea of scaling this bonus depending on points, however.  I 
think it would be better if we could scale the discount but also limit it 
to some sane value.
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