Re: [v7 5/5] mm, oom: cgroup v2 mount option to disable cgroup-aware OOM killer

[Roman Gushchin <guro@fb.com>]

On Tue, Sep 05, 2017 at 05:12:51PM +0200, Michal Hocko wrote:
> 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.

It doesn't look as a nice default interface.

> 
> > > 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.

I agree. And if so, why do we need a new interface for an useless feature?

> 
> > 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.

I'd agree with you, if there are use cases (excluding pure legacy),
when the per-process algorithm is preferable over the cgroup-aware OOM.
I really doubt, and hope, that with oom_priorities the suggested algorithm
should cover almost all reasonable use cases.

Thanks!

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