linux-kernel - Re: [v8 3/4] mm, oom: add cgroup v2 mount option for cgroup-aware OOM killer

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Message-ID: <alpine.DEB.2.10.1709121319040.62551@chino.kir.corp.google.com>
Date:   Tue, 12 Sep 2017 13:23:00 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Roman Gushchin <guro@...com>
cc:     linux-mm@...ck.org, Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, kernel-team@...com,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v8 3/4] mm, oom: add cgroup v2 mount option for cgroup-aware
 OOM killer

On Tue, 12 Sep 2017, Roman Gushchin wrote:

> > 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.
> 

If it absolutely must be a mount option, then I would agree it should be 
opt-in so that it's known what is being changed rather than changing how 
selection was done in the past and requiring legacy users to now mount in 
a new way.

I'd be interested to hear Tejun's comments, however, about whether we want 
to add controller specific mount options like this instead of a tunable at 
the root level, for instance, that controls victim selection and would be 
isolated to the memory cgroup controller as opposed to polluting mount 
options.