Re: [v5 2/4] mm, oom: cgroup-aware OOM killer

Roman Gushchin
2017-08-23T16:20:31Z

Hi Johannes!

Thank you for review!

I do agree with most of the comments, and I will address them in v6.
I'll post it soon.

Please, find some comments below.

On Tue, Aug 22, 2017 at 01:03:44PM -0400, Johannes Weiner wrote:
> Hi Roman,
>
> great work! This looks mostly good to me now. Below are some nitpicks
> concerning naming and code layout, but nothing major.
>
> > +
> > + css_task_iter_start(&memcg->css, 0, &it);
> > + while ((task = css_task_iter_next(&it))) {
> > + /*
> > + * If there are no tasks, or all tasks have oom_score_adj set
> > + * to OOM_SCORE_ADJ_MIN and oom_kill_all_tasks is not set,
> > + * don't select this memory cgroup.
> > + */
> > + if (!elegible &&
> > + (memcg->oom_kill_all_tasks ||
> > + task->signal->oom_score_adj != OOM_SCORE_ADJ_MIN))
> > + elegible = 1;
>
> This is a little awkward to read. How about something like this:
>
> /*
> * When killing individual tasks, we respect OOM score adjustments:
> * at least one task in the group needs to be killable for the group
> * to be oomable.
> *
> * Also check that previous OOM kills have finished, and abort if
> * there are any pending OOM victims.
> */
> oomable = memcg->oom_kill_all_tasks;
> while ((task = css_task_iter_next(&it))) {
> if (!oomable && task->signal_oom_score_adj != OOM_SCORE_ADJ_MIN)
> oomable = 1;
>
> > + if (tsk_is_oom_victim(task) &&
> > + !test_bit(MMF_OOM_SKIP, &task->signal->oom_mm->flags)) {
> > + elegible = -1;
> > + break;
> > + }
> > + }
> > + css_task_iter_end(&it);

We ignore oom_score_adj if oom_kill_all_tasks is set, it's
not reflected in your version. Anyway, I've moved the comments block
outside and rephrased it to make more clear.

>
> etc.
>
> > +
> > + return elegible > 0 ? memcg_oom_badness(memcg, nodemask) : elegible;
>
> I find these much easier to read if broken up, even if it's more LOC:
>
> if (eligible <= 0)
> return eligible;
>
> return memcg_oom_badness(memcg, nodemask);
>
> > +static void select_victim_memcg(struct mem_cgroup *root, struct oom_control *oc)
> > +{
> > + struct mem_cgroup *iter, *parent;
> > +
> > + for_each_mem_cgroup_tree(iter, root) {
> > + if (memcg_has_children(iter)) {
> > + iter->oom_score = 0;
> > + continue;
> > + }
> > +
> > + iter->oom_score = oom_evaluate_memcg(iter, oc->nodemask);
> > + if (iter->oom_score == -1) {
>
> Please add comments to document the special returns. Maybe #defines
> would be clearer, too.
>
> > + oc->chosen_memcg = (void *)-1UL;
> > + mem_cgroup_iter_break(root, iter);
> > + return;
> > + }
> > +
> > + if (!iter->oom_score)
> > + continue;
>
> Same here.
>
> Maybe a switch would be suitable to handle the abort/no-score cases.

Not sure about switch/defines, but I've added several comment blocks
to describe possible return values, as well as their handling.
Hope, it will be enough.

> > static int memory_events_show(struct seq_file *m, void *v)
> > {
> > struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(m));
> > @@ -5310,6 +5512,12 @@ static struct cftype memory_files[] = {
> > .write = memory_max_write,
> > },
> > {
> > + .name = "oom_kill_all_tasks",
> > + .flags = CFTYPE_NOT_ON_ROOT,
> > + .seq_show = memory_oom_kill_all_tasks_show,
> > + .write = memory_oom_kill_all_tasks_write,
> > + },
>
> This name is quite a mouthful and reminiscent of the awkward v1
> interface names. It doesn't really go well with the v2 names.
>
> How about memory.oom_group?

I'd prefer to have something more obvious. I've renamed
memory.oom_kill_all_tasks to memory.oom_kill_all, which was earlier suggested
by Vladimir. Are you ok with it?

Thanks!