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

[Peter Zijlstra]

On Wed, Jul 19, 2017 at 10:20:29AM +0100, Juri Lelli wrote:
> 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.

I don't think that quite works out. Given that the frequency selection
will never quite end up at exactly the same fraction (if the hardware
listens to your requests at all).

Also, by not scaling the GRUB stuff, don't you run the risk of
attempting to hand out more idle time than there actually is?