Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in scale_rt_capacity

[Luca Abeni <luca.abeni@unitn.it>]

On Mon, 14 Dec 2015 16:56:17 +0100
Vincent Guittot <vincent.guittot@linaro.org> wrote:
[...]
> >> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> >> index 08858d1..e44c6be 100644
> >> --- a/kernel/sched/sched.h
> >> +++ b/kernel/sched/sched.h
> >> @@ -519,6 +519,8 @@ struct dl_rq {
> >>  #else
> >>       struct dl_bw dl_bw;
> >>  #endif
> >> +     /* This is the "average utilization" for this runqueue */
> >> +     s64 avg_bw;
> >>  };
> >
> > So I don't think this is right. AFAICT this projects the WCET as the
> > amount of time actually used by DL. This will, under many
> > circumstances, vastly overestimate the amount of time actually
> > spend on it. Therefore unduly pessimisme the fair capacity of this
> > CPU.
> 
> I agree that if the WCET is far from reality, we will underestimate
> available capacity for CFS. Have you got some use case in mind which
> overestimates the WCET ?
> If we can't rely on this parameters to evaluate the amount of capacity
> used by deadline scheduler on a core, this will imply that we can't
> also use it for requesting capacity to cpufreq and we should fallback
> on a monitoring mechanism which reacts to a change instead of
> anticipating it.
I think a more "theoretically sound" approach would be to track the
_active_ utilisation (informally speaking, the sum of the utilisations
of the tasks that are actually active on a core - the exact definition
of "active" is the trick here).
As done, for example, here:
https://github.com/lucabe72/linux-reclaiming/tree/track-utilisation-v2
(in particular, see
https://github.com/lucabe72/linux-reclaiming/commit/49fc786a1c453148625f064fa38ea538470df55b
)
I understand this approach might look too complex... But I think it is
much less pessimistic while still being "safe".
If there is something that I can do to make that code more acceptable,
let me know.


			Luca