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

[Shakeel Butt]

On Tue, Oct 31, 2017 at 9:40 AM, Johannes Weiner <hannes@xxxxxxxxxxx> 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?

> To be consistent with that rule, we might want to disallow
> the creation of child groups once a cgroup has local memory charges.
>
> It's trivial to change the setup sequence to create the leaf cgroup
> first, then launch the workload from within.
>

Only if cgroup hierarchy is centrally controller and each task's whole
hierarchy is known in advance.

> Either way, this is nothing specific about the OOM killer.