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

[Shakeel Butt <shakeelb@google.com>]

> +
> +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?

> +               score = oom_evaluate_memcg(iter, oc->nodemask, oc->totalpages);
> +
> +               /*
> +                * Ignore empty and non-eligible memory cgroups.
> +                */
> +               if (score == 0)
> +                       continue;
> +
> +               /*
> +                * If there are inflight OOM victims, we don't need
> +                * to look further for new victims.
> +                */
> +               if (score == -1) {
> +                       oc->chosen_memcg = INFLIGHT_VICTIM;
> +                       mem_cgroup_iter_break(root, iter);
> +                       break;
> +               }
> +
> +               if (score > oc->chosen_points) {
> +                       oc->chosen_points = score;
> +                       oc->chosen_memcg = iter;
> +               }
> +       }
> +
> +       if (oc->chosen_memcg && oc->chosen_memcg != INFLIGHT_VICTIM)
> +               css_get(&oc->chosen_memcg->css);
> +
> +       rcu_read_unlock();
> +}