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

[Michal Hocko <mhocko@kernel.org>]

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.

> > 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.

> FB is moving away from limits setting, so, not sure if they have
> thought of these cases.
> 
> BTW for such use-cases, shouldn't we be taking the memcg's oom_lock?

This is a good question. I would have to go and double check the code
but I suspect that this is an omission.
-- 
Michal Hocko
SUSE Labs