Date: Tue, 12 Sep 2017 21:01:15 +0100
From: Roman Gushchin <guro@fb.com>
To: David Rientjes <rientjes@google.com>
CC: <linux-mm@kvack.org>, Michal Hocko <mhocko@kernel.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Andrew Morton <akpm@linux-foundation.org>, Tejun Heo <tj@kernel.org>,
        <kernel-team@fb.com>, <cgroups@vger.kernel.org>,
        <linux-doc@vger.kernel.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [v8 3/4] mm, oom: add cgroup v2 mount option for cgroup-aware
 OOM killer
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On Mon, Sep 11, 2017 at 01:48:39PM -0700, David Rientjes wrote:
> On Mon, 11 Sep 2017, Roman Gushchin wrote:
> 
> > Add a "groupoom" cgroup v2 mount option to enable the cgroup-aware
> > OOM killer. If not set, the OOM selection is performed in
> > a "traditional" per-process way.
> > 
> > The behavior can be changed dynamically by remounting the cgroupfs.
> 
> I can't imagine that Tejun would be happy with a new mount option, 
> especially when it's not required.
> 
> OOM behavior does not need to be defined at mount time and for the entire 
> hierarchy.  It's possible to very easily implement a tunable as part of 
> mem cgroup that is propagated to descendants and controls the oom scoring 
> behavior for that hierarchy.  It does not need to be system wide and 
> affect scoring of all processes based on which mem cgroup they are 
> attached to at any given time.

No, I don't think that mixing per-cgroup and per-process OOM selection
algorithms is a good idea.

So, there are 3 reasonable options:
1) boot option
2) sysctl
3) cgroup mount option

I believe, 3) is better, because it allows changing the behavior dynamically,
and explicitly depends on v2 (what sysctl lacks).

So, the only question is should it be opt-in or opt-out option.
Personally, I would prefer opt-out, but Michal has a very strong opinion here.

Thanks!