Re: [PATCH v5 09/14] sched: Add over-utilization/tipping point indicator

[Vincent Guittot]

On Thu, 2 Aug 2018 at 16:14, Quentin Perret <quentin.perret@xxxxxxx> wrote:
>
> On Thursday 02 Aug 2018 at 15:48:01 (+0200), Vincent Guittot wrote:
> > On Thu, 2 Aug 2018 at 15:19, Quentin Perret <quentin.perret@xxxxxxx> wrote:
> > >
> > > On Thursday 02 Aug 2018 at 15:08:01 (+0200), Peter Zijlstra wrote:
> > > > On Thu, Aug 02, 2018 at 02:03:38PM +0100, Quentin Perret wrote:
> > > > > On Thursday 02 Aug 2018 at 14:26:29 (+0200), Peter Zijlstra wrote:
> > > > > > On Tue, Jul 24, 2018 at 01:25:16PM +0100, Quentin Perret wrote:
> > > > > > > @@ -5100,8 +5118,17 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> > > > > > >                 update_cfs_group(se);
> > > > > > >         }
> > > > > > >
> > > > > > > -       if (!se)
> > > > > > > +       if (!se) {
> > > > > > >                 add_nr_running(rq, 1);
> > > > > > > +               /*
> > > > > > > +                * The utilization of a new task is 'wrong' so wait for it
> > > > > > > +                * to build some utilization history before trying to detect
> > > > > > > +                * the overutilized flag.
> > > > > > > +                */
> > > > > > > +               if (flags & ENQUEUE_WAKEUP)
> > > > > > > +                       update_overutilized_status(rq);
> > > > > > > +
> > > > > > > +       }
> > > > > > >
> > > > > > >         hrtick_update(rq);
> > > > > > >  }
> > > > > >
> > > > > > That is a somewhat dodgy hack. There is no guarantee what so ever that
> > > > > > when the task wakes next its history is any better. The comment doesn't
> > > > > > reflect this I feel.
> > > > >
> > > > > AFAICT the main use-case here is to avoid re-enabling the load balance
> > > > > and ruining all the task placement because of a tiny task. I don't
> > > > > really see how we can do that differently ...
> > > >
> > > > Sure I realize that.. but it doesn't completely avoid it. Suppose this
> > > > new task instantly blocks and wakes up again. Then its util signal will
> > > > be exactly what you didn't want but we'll account it and cause the above
> > > > scenario you wanted to avoid.
> > >
> > > That is true. ... I also realize now that this patch was written long
> > > before util_est, and that also has an impact here, especially in the
> > > scenario you described where the task blocks. So any wake-up after the
> > > first enqueue will risk to overutilize the system, even if the task
> > > blocked for ages.
> > >
> > > Hmm ...
> >
> > Does a init value set to 0 for util_avg for newly created task can
> > help in EAS in this case ?
> > Current initial value is computed to prevent packing newly created
> > tasks on same CPUs because it hurts performance of some benches. In
> > fact it somehow assumes that newly created task will use significant
> > part of the remaining capacity of a CPU and want to spread tasks. In
> > EAS case, it seems that it prefer to assume that  newly created task
> > are small and we can pack them and wait a bit to make sure the new
> > task will be a big task and will overload the CPU
>
> Good point, setting the util_avg to 0 for new tasks should help
> filtering out those tiny tasks too. And that would match with the idea
> of letting tasks build their history before looking at their util_avg ...
>
> But there is one difference w.r.t frequency selection. The current code
> won't mark the system overutilized, but will let sugov raise the
> frequency when a new task is enqueued. So in case of a fork bomb, we

If the initial value of util_avg is 0, we should not have any impact
on the util_avg of the cfs rq on which the task is attached, isn't it
? so this should not impact both the over utilization state and the
frequency selected by sugov or I'm missing something ?
Then, select_task_rq_fair is called for a new task but util_avg is
still 0 at that time in the current code so you will have consistent
util_avg of the new task before and after calling
find_energy_efficient_cpu

> sort of fallback on the existing mainline strategy for both task
> placement (because forkees don't go in find_energy_efficient_cpu) and
> frequency selection. And I would argue this is the right thing to do
> since EAS can't really help in this case.
>
> Thoughts ?
>
> Thanks,
> Quentin