Re: [v10 3/6] mm, oom: cgroup-aware OOM killer

[Roman Gushchin <guro@fb.com>]

On Wed, Oct 04, 2017 at 02:24:26PM -0700, Shakeel Butt wrote:
> >> > +               if (memcg_has_children(iter))
> >> > +                       continue;
> >>
> >> && iter != root_mem_cgroup ?
> >
> > Oh, sure. I had a stupid bug in my test script, which prevented me from
> > catching this. Thanks!
> >
> > This should fix the problem.
> > --
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 2e82625bd354..b3848bce4c86 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2807,7 +2807,8 @@ static void select_victim_memcg(struct mem_cgroup *root, struct oom_control *oc)
> >                  * We don't consider non-leaf non-oom_group memory cgroups
> >                  * as OOM victims.
> >                  */
> > -               if (memcg_has_children(iter) && !mem_cgroup_oom_group(iter))
> > +               if (memcg_has_children(iter) && iter != root_mem_cgroup &&
> > +                   !mem_cgroup_oom_group(iter))
> >                         continue;
> 
> I think you are mixing the 3rd and 4th patch. The root_mem_cgroup
> check should be in 3rd while oom_group stuff should be in 4th.
>

Right. This "patch" should fix them both, it was just confusing to
send two patches. I'll split it before final landing.

> 
> >>
> >> Shouldn't there be a CSS_ONLINE check? Also instead of css_get at the
> >> end why not css_tryget_online() here and css_put for the previous
> >> selected one.
> >
> > Hm, why do we need to check this? I do not see, how we can choose
> > an OFFLINE memcg as a victim, tbh. Please, explain the problem.
> >
> 
> Sorry about the confusion. There are two things. First, should we do a
> css_get on the newly selected memcg within the for loop when we still
> have a reference to it?

We're holding rcu_read_lock, it should be enough. We're bumping css counter
just before releasing rcu lock.

> 
> Second, for the OFFLINE memcg, you are right oom_evaluate_memcg() will
> return 0 for offlined memcgs. Maybe no need to call
> oom_evaluate_memcg() for offlined memcgs.

Sounds like a good optimization, which can be done on top of the current
patchset.

Thank you!