Re: [RFC PATCH v1 8/8] sched/deadline: make bandwidth enforcement scale-invariant

Juri Lelli
2017-07-19T09:20:39Z

On 19/07/17 09:21, Peter Zijlstra wrote:
> On Wed, Jul 05, 2017 at 09:59:05AM +0100, Juri Lelli wrote:
> > @@ -1156,9 +1157,26 @@ static void update_curr_dl(struct rq *rq)
> > if (unlikely(dl_entity_is_special(dl_se)))
> > return;
> >
> > - if (unlikely(dl_se->flags & SCHED_FLAG_RECLAIM))
> > - delta_exec = grub_reclaim(delta_exec, rq, &curr->dl);
> > - dl_se->runtime -= delta_exec;
> > + /*
> > + * For tasks that participate in GRUB, we implement GRUB-PA: the
> > + * spare reclaimed bandwidth is used to clock down frequency.
> > + *
> > + * For the others, we still need to scale reservation parameters
> > + * according to current frequency and CPU maximum capacity.
> > + */
> > + if (unlikely(dl_se->flags & SCHED_FLAG_RECLAIM)) {
> > + scaled_delta_exec = grub_reclaim(delta_exec,
> > + rq,
> > + &curr->dl);
> > + } else {
> > + unsigned long scale_freq = arch_scale_freq_capacity(cpu);
> > + unsigned long scale_cpu = arch_scale_cpu_capacity(NULL, cpu);
> > +
> > + scaled_delta_exec = cap_scale(delta_exec, scale_freq);
> > + scaled_delta_exec = cap_scale(scaled_delta_exec, scale_cpu);
> > + }
> > +
> > + dl_se->runtime -= scaled_delta_exec;
> >
>
> This I don't get...

Considering that we use GRUB's active utilization to drive clock
frequency selection, rationale is that GRUB tasks don't need any special
scaling, as their delta_exec is already scaled according to GRUB rules.
OTOH, normal tasks need to have their runtime (delta_exec) explicitly
scaled considering current frequency (and CPU max capacity), otherwise
they are going to receive less runtime than granted at AC, when
frequency is reduced.