Re: [v8 0/4] cgroup-aware OOM killer

Michal Hocko
2017-09-14T13:34:07Z

On Wed 13-09-17 13:46:08, David Rientjes wrote:
> On Wed, 13 Sep 2017, Michal Hocko wrote:
>
> > > > This patchset makes the OOM killer cgroup-aware.
> > > >
> > > > v8:
> > > > - Do not kill tasks with OOM_SCORE_ADJ -1000
> > > > - Make the whole thing opt-in with cgroup mount option control
> > > > - Drop oom_priority for further discussions
> > >
> > > Nack, we specifically require oom_priority for this to function correctly,
> > > otherwise we cannot prefer to kill from low priority leaf memcgs as
> > > required.
> >
> > While I understand that your usecase might require priorities I do not
> > think this part missing is a reason to nack the cgroup based selection
> > and kill-all parts. This can be done on top. The only important part
> > right now is the current selection semantic - only leaf memcgs vs. size
> > of the hierarchy). I strongly believe that comparing only leaf memcgs
> > is more straightforward and it doesn't lead to unexpected results as
> > mentioned before (kill a small memcg which is a part of the larger
> > sub-hierarchy).
> >
>
> The problem is that we cannot enable the cgroup-aware oom killer and
> oom_group behavior because, without oom priorities, we have no ability to
> influence the cgroup that it chooses. It is doing two things: providing
> more fairness amongst cgroups by selecting based on cumulative usage
> rather than single large process (good!), and effectively is removing all
> userspace control of oom selection (bad). We want the former, but it
> needs to be coupled with support so that we can protect vital cgroups,
> regardless of their usage.

I understand that your usecase needs a more fine grained control over
the selection but that alone is not a reason to nack the implementation
which doesn't provide it (yet).

> It is certainly possible to add oom priorities on top before it is merged,
> but I don't see why it isn't part of the patchset.

Because the semantic of the priority for non-leaf memcgs is not fully
clear and I would rather have the core of the functionality merged
before this is sorted out.

> We need it before its
> merged to avoid users playing with /proc/pid/oom_score_adj to prevent any
> killing in the most preferable memcg when they could have simply changed
> the oom priority.

I am sorry but I do not really understand your concern. Are you
suggesting that users would start oom disable all tasks in a memcg to
give it a higher priority? Even if that was the case why should such an
abuse be a blocker for generic memcg aware oom killer being merged?
--
Michal Hocko
SUSE Labs