Date: Tue, 5 Sep 2017 17:12:51 +0200
From: Michal Hocko <mhocko@kernel.org>
To: Roman Gushchin <guro@fb.com>
Cc: linux-mm@kvack.org, Vladimir Davydov <vdavydov.dev@gmail.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        David Rientjes <rientjes@google.com>,
        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: [v7 5/5] mm, oom: cgroup v2 mount option to disable cgroup-aware
 OOM killer
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On Tue 05-09-17 15:30:21, Roman Gushchin wrote:
> On Tue, Sep 05, 2017 at 03:44:12PM +0200, Michal Hocko wrote:
[...]
> > Why is this an opt out rather than opt-in? IMHO the original oom logic
> > should be preserved by default and specific workloads should opt in for
> > the cgroup aware logic. Changing the global behavior depending on
> > whether cgroup v2 interface is in use is more than unexpected and IMHO
> > wrong approach to take. I think we should instead go with 
> > oom_strategy=[alloc_task,biggest_task,cgroup]
> > 
> > we currently have alloc_task (via sysctl_oom_kill_allocating_task) and
> > biggest_task which is the default. You are adding cgroup and the more I
> > think about the more I agree that it doesn't really make sense to try to
> > fit thew new semantic into the existing one (compare tasks to kill-all
> > memcgs). Just introduce a new strategy and define a new semantic from
> > scratch. Memcg priority and kill-all are a natural extension of this new
> > strategy. This will make the life easier and easier to understand by
> > users.
> > 
> > Does that make sense to you?
> 
> Absolutely.
> 
> The only thing: I'm not sure that we have to preserve the existing logic
> as default option. For most users (except few very specific usecases),
> it should be at least as good, as the existing one.

But this is really an unexpected change. Users even might not know that
they are using cgroup v2 and memcg is in use.

> Making it opt-in means that corresponding code will be executed only
> by few users, who cares.

Yeah, which is the way we should introduce new features no?

> Then we should probably hide corresponding
> cgroup interface (oom_group and oom_priority knobs) by default,
> and it feels as unnecessary complication and is overall against
> cgroup v2 interface design.

Why. If we care enough, we could simply return EINVAL when those knobs
are written while the corresponding strategy is not used.

> > I think we should instead go with
> > oom_strategy=[alloc_task,biggest_task,cgroup]
> 
> It would be a really nice interface; although I've no idea how to implement it:
> "alloc_task" is an existing sysctl, which we have to preserve;

I would argue that we should simply deprecate and later drop the sysctl.
I _strongly_ suspect anybody is using this. If yes it is not that hard
to change the kernel command like rather than select the sysctl. The
deprecation process would be
	- warn when somebody writes to the sysctl and check both boot
	  and sysctl values
	[ wait some time ]
	- keep the sysctl but return EINVAL
	[ wait some time ]
	- remove the sysctl

> while "cgroup" depends on cgroup v2.

Which is not a big deal either. Simply fall back to default if there are
no cgroup v2. The implementation would have essentially the same effect
because there won't be any kill-all cgroups and so we will select the
largest task.
-- 
Michal Hocko
SUSE Labs