Re: [PATCH] doc: memcontrol: add description for oom_kill

[Yang Shi]

On Mon, Mar 1, 2021 at 4:24 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Fri 26-02-21 11:19:51, Yang Shi wrote:
> > On Fri, Feb 26, 2021 at 8:42 AM Yang Shi <shy828301@xxxxxxxxx> wrote:
> > >
> > > On Thu, Feb 25, 2021 at 11:30 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > > >
> > > > On Thu 25-02-21 18:12:54, Yang Shi wrote:
> > > > > When debugging an oom issue, I found the oom_kill counter of memcg is
> > > > > confusing.  At the first glance without checking document, I thought it
> > > > > just counts for memcg oom, but it turns out it counts both global and
> > > > > memcg oom.
> > > >
> > > > Yes, this is the case indeed. The point of the counter was to count oom
> > > > victims from the memcg rather than matching that to the source of the
> > > > oom. Rememeber that this could have been a memcg oom up in the
> > > > hierarchy as well. Counting victims on the oom origin could be equally
> > >
> > > Yes, it is updated hierarchically on v2, but not on v1. I'm supposed
> > > this is because v1 may work in non-hierarchcal mode? If this is the
> > > only reason we may be able to remove this to get aligned with v2 since
> > > non-hierarchal mode is no longer supported.
> >
> > BTW, having the counter recorded hierarchically may help out one of
> > our usecases. We want to monitor the oom_kill for some services, but
> > systemd would wipe out the cgroup if the service is oom killed then
> > restart the service from scratch (it means create a brand new cgroup
> > with the same name). So this systemd behavior makes the counter
> > useless if it is not recorded hierarchically.
>
> Just to make sure I understand correctly. You have a setup where memcg
> for a service has a hard limit configured and it is destroyed when oom
> happens inside that memcg. A new instance is created at the same place
> of the hierarchy with a new memcg. Your problem is that the oom killed
> memcg will not be recorded in its parent oom event and the information
> will get lost with the torn down memcg. Correct?

Yes. But global oom instead of memcg oom.

>
> If yes then how do you tell which of the child cgroup was killed from
> the parent counter? Or is there only a single child?

Not only a single child, but our case is that oom-killed child
consumes 90% memory, then global oom would kill it. This definitely
doesn't prevent from accounting oom from other children, but we don't
have to have a very accurate counter and in our case we can tell 99%
oom kill happens with that specific memcg.

>
> Anyway, cgroup v2 will offer the hierarchical behavior. Do you have any
> strong reasons that you cannot use v2?

I do prefer to migrate to cgroup v2 personally. But it incurs
significant work for orchestration tools, infrastructure
configuration, monitoring tools, etc which are out of my control.

> --
> Michal Hocko
> SUSE Labs