Re: [RFC PATCH 2/2] memcg: do not report racy no-eligible OOM tasks

From: Michal Hocko
Date: Tue Oct 23 2018 - 08:10:59 EST


On Tue 23-10-18 13:42:46, Michal Hocko wrote:
> On Tue 23-10-18 10:01:08, Tetsuo Handa wrote:
> > Michal Hocko wrote:
> > > On Mon 22-10-18 20:45:17, Tetsuo Handa wrote:
> > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > > > index e79cb59552d9..a9dfed29967b 100644
> > > > > --- a/mm/memcontrol.c
> > > > > +++ b/mm/memcontrol.c
> > > > > @@ -1380,10 +1380,22 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
> > > > > .gfp_mask = gfp_mask,
> > > > > .order = order,
> > > > > };
> > > > > - bool ret;
> > > > > + bool ret = true;
> > > > >
> > > > > mutex_lock(&oom_lock);
> > > > > +
> > > > > + /*
> > > > > + * multi-threaded tasks might race with oom_reaper and gain
> > > > > + * MMF_OOM_SKIP before reaching out_of_memory which can lead
> > > > > + * to out_of_memory failure if the task is the last one in
> > > > > + * memcg which would be a false possitive failure reported
> > > > > + */
> > > > > + if (tsk_is_oom_victim(current))
> > > > > + goto unlock;
> > > > > +
> > > >
> > > > This is not wrong but is strange. We can use mutex_lock_killable(&oom_lock)
> > > > so that any killed threads no longer wait for oom_lock.
> > >
> > > tsk_is_oom_victim is stronger because it doesn't depend on
> > > fatal_signal_pending which might be cleared throughout the exit process.
> > >
> >
> > I still want to propose this. No need to be memcg OOM specific.
>
> Well, I maintain what I've said [1] about simplicity and specific fix
> for a specific issue. Especially in the tricky code like this where all
> the consequences are far more subtle than they seem to be.
>
> This is obviously a matter of taste but I don't see much point discussing
> this back and forth for ever. Unless there is a general agreement that
> the above is less appropriate then I am willing to consider a different
> change but I simply do not have energy to nit pick for ever.
>
> [1] http://lkml.kernel.org/r/20181022134315.GF18839@xxxxxxxxxxxxxx

In other words. Having a memcg specific fix means, well, a memcg
maintenance burden. Like any other memcg specific oom decisions we
already have. So are you OK with that Johannes or you would like to see
a more generic fix which might turn out to be more complex?
--
Michal Hocko
SUSE Labs