Re: [PATCH] memcg: oom: ignore oom warnings from memory.max

[Shakeel Butt <shakeelb@google.com>]

On Mon, May 4, 2020 at 7:11 AM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Mon 04-05-20 06:54:40, Shakeel Butt wrote:
> > On Sun, May 3, 2020 at 11:56 PM Michal Hocko <mhocko@kernel.org> wrote:
> > >
> > > On Thu 30-04-20 11:27:12, Shakeel Butt wrote:
> > > > Lowering memory.max can trigger an oom-kill if the reclaim does not
> > > > succeed. However if oom-killer does not find a process for killing, it
> > > > dumps a lot of warnings.
> > >
> > > It shouldn't dump much more than the regular OOM report AFAICS. Sure
> > > there is "Out of memory and no killable processes..." message printed as
> > > well but is that a real problem?
> > >
> > > > Deleting a memcg does not reclaim memory from it and the memory can
> > > > linger till there is a memory pressure. One normal way to proactively
> > > > reclaim such memory is to set memory.max to 0 just before deleting the
> > > > memcg. However if some of the memcg's memory is pinned by others, this
> > > > operation can trigger an oom-kill without any process and thus can log a
> > > > lot un-needed warnings. So, ignore all such warnings from memory.max.
> > >
> > > OK, I can see why you might want to use memory.max for that purpose but
> > > I do not really understand why the oom report is a problem here.
> >
> > It may not be a problem for an individual or small scale deployment
> > but when "sweep before tear down" is the part of the workflow for
> > thousands of machines cycling through hundreds of thousands of cgroups
> > then we can potentially flood the logs with not useful dumps and may
> > hide (or overflow) any useful information in the logs.
>
> If you are doing this in a large scale and the oom report is really a
> problem then you shouldn't be resetting hard limit to 0 in the first
> place.
>

I think I have pretty clearly described why we want to reset the hard
limit to 0, so, unless there is an alternative I don't see why we
should not be doing this.

> > > memory.max can trigger the oom kill and user should be expecting the oom
> > > report under that condition. Why is "no eligible task" so special? Is it
> > > because you know that there won't be any tasks for your particular case?
> > > What about other use cases where memory.max is not used as a "sweep
> > > before tear down"?
> >
> > What other such use-cases would be? The only use-case I can envision
> > of adjusting limits dynamically of a live cgroup are resource
> > managers. However for cgroup v2, memory.high is the recommended way to
> > limit the usage, so, why would resource managers be changing
> > memory.max instead of memory.high? I am not sure. What do you think?
>
> There are different reasons to use the hard limit. Mostly to contain
> potential runaways. While high limit might be a sufficient measure to
> achieve that as well the hard limit is the last resort. And it clearly
> has the oom killer semantic so I am not really sure why you are
> comparing the two.
>

I am trying to see if "no eligible task" is really an issue and should
be warned for the "other use cases". The only real use-case I can
think of are resource managers adjusting the limit dynamically. I
don't see "no eligible task" a concerning reason for such use-case. If
you have some other use-case please do tell.

Shakeel