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

[Quentin Perret <quentin.perret@arm.com>]

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@arm.com> 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
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