linux-kernel - Re: [RFC PATCH 1/7] PM: Introduce em_pd_get_higher

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Date:   Thu, 16 May 2019 14:22:50 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Quentin Perret <quentin.perret@....com>
Cc:     douglas.raillard@....com, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, mingo@...hat.com, peterz@...radead.org,
        dietmar.eggemann@....com
Subject: Re: [RFC PATCH 1/7] PM: Introduce em_pd_get_higher_freq()

On 16-May 14:01, Quentin Perret wrote:
> On Thursday 16 May 2019 at 13:42:00 (+0100), Patrick Bellasi wrote:
> > > +static inline unsigned long em_pd_get_higher_freq(struct em_perf_domain *pd,
> > > +	unsigned long min_freq, unsigned long cost_margin)
> > > +{
> > > +	unsigned long max_cost = 0;
> > > +	struct em_cap_state *cs;
> > > +	int i;
> > > +
> > > +	if (!pd)
> > > +		return min_freq;
> > > +
> > > +	/* Compute the maximum allowed cost */
> > > +	for (i = 0; i < pd->nr_cap_states; i++) {
> > > +		cs = &pd->table[i];
> > > +		if (cs->frequency >= min_freq) {
> > > +			max_cost = cs->cost + (cs->cost * cost_margin) / 1024;
> >                                                                          ^^^^
> > ... end here we should probably better use SCHED_CAPACITY_SCALE
> > instead of hard-coding in values, isn't it?
> 
> I'm not sure to agree. This isn't part of the scheduler per se, and the
> cost thing isn't in units of capacity, but in units of power, so I don't
> think SCHED_CAPACITY_SCALE is correct here.

Right, I get the units do not match and it would not be elegant to use
it here...

> But I agree these hard coded values (that one, and the 512 in one of the
> following patches) could use some motivation :-)

... ultimately SCHED_CAPACITY_SCALE is just SCHED_FIXEDPOINT_SCALE,
which is adimensional. Perhaps we should use that or yet another alias
for the same.

> Thanks,
> Quentin

-- 
#include <best/regards.h>

Patrick Bellasi