Re: [RESEND v12 3/6] mm, oom: cgroup-aware OOM killer

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Oct 31, 2017 at 10:50:43AM -0700, Shakeel Butt wrote:
> On Tue, Oct 31, 2017 at 9:40 AM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> > On Tue, Oct 31, 2017 at 08:04:19AM -0700, Shakeel Butt wrote:
> >> > +
> >> > +static void select_victim_memcg(struct mem_cgroup *root, struct oom_control *oc)
> >> > +{
> >> > +       struct mem_cgroup *iter;
> >> > +
> >> > +       oc->chosen_memcg = NULL;
> >> > +       oc->chosen_points = 0;
> >> > +
> >> > +       /*
> >> > +        * The oom_score is calculated for leaf memory cgroups (including
> >> > +        * the root memcg).
> >> > +        */
> >> > +       rcu_read_lock();
> >> > +       for_each_mem_cgroup_tree(iter, root) {
> >> > +               long score;
> >> > +
> >> > +               if (memcg_has_children(iter) && iter != root_mem_cgroup)
> >> > +                       continue;
> >> > +
> >>
> >> Cgroup v2 does not support charge migration between memcgs. So, there
> >> can be intermediate nodes which may contain the major charge of the
> >> processes in their leave descendents. Skipping such intermediate nodes
> >> will kind of protect such processes from oom-killer (lower on the list
> >> to be killed). Is it ok to not handle such scenario? If yes, shouldn't
> >> we document it?
> >
> > Tasks cannot be in intermediate nodes, so the only way you can end up
> > in a situation like this is to start tasks fully, let them fault in
> > their full workingset, then create child groups and move them there.
> >
> > That has attribution problems much wider than the OOM killer: any
> > local limits you would set on a leaf cgroup like this ALSO won't
> > control the memory of its tasks - as it's all sitting in the parent.
> >
> > We created the "no internal competition" rule exactly to prevent this
> > situation.
> 
> Rather than the "no internal competition" restriction I think "charge
> migration" would have resolved that situation? Also "no internal
> competition" restriction (I am assuming 'no internal competition' is
> no tasks in internal nodes, please correct me if I am wrong) has made
> "charge migration" hard to implement and thus not added in cgroup v2.
> 
> I know this is parallel discussion and excuse my ignorance, what are
> other reasons behind "no internal competition" specifically for memory
> controller?

Sorry, but this is completely off-topic.

The rationale for this decisions is in Documentation/cgroup-v2.txt.