Re: [RFC PATCH v3 1/6] PM: Introduce em_pd_get_higher_freq()

[Douglas Raillard]

Hi Dietmar,

On 10/17/19 10:58 AM, Dietmar Eggemann wrote:
> On 11/10/2019 15:44, Douglas RAILLARD wrote:
>
> [...]
>
> > diff --git a/include/linux/energy_model.h b/include/linux/energy_model.h
> > index d249b88a4d5a..dd6a35f099ea 100644
> > --- a/include/linux/energy_model.h
> > +++ b/include/linux/energy_model.h
> > @@ -159,6 +159,53 @@ static inline int em_pd_nr_cap_states(struct em_perf_domain *pd)
> >   	return pd->nr_cap_states;
> >   }
> >   
> > +#define EM_COST_MARGIN_SCALE 1024U
> > +
> > +/**
> > + * em_pd_get_higher_freq() - Get the highest frequency that does not exceed the
> > + * given cost margin compared to min_freq
> > + * @pd		: performance domain for which this must be done
> > + * @min_freq	: minimum frequency to return
> > + * @cost_margin	: allowed margin compared to min_freq, on the
> > + *		  EM_COST_MARGIN_SCALE scale.
> > + *
> > + * Return: the chosen frequency, guaranteed to be at least as high as min_freq.
> > + */
> > +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) / EM_COST_MARGIN_SCALE;
>
> Maybe you could mention in the header that this is the place where the
> algorithm could be tuned. (even though it doesn't offer any tuning
> interface, which is a good thing).

I'm not sure what you mean, the patch "title" contains "em_pd_get_higher_freq()", should it
also mention where exactly inside the function the margin logic is implemented ?

> > +			break;
> > +		}
> > +	}
> > +
> > +	/* Find the highest frequency that will not exceed the cost margin */
> > +	for (i = pd->nr_cap_states-1; i >= 0; i--) {
> > +		cs = &pd->table[i];
> > +		if (cs->cost <= max_cost)
> > +			return cs->frequency;
> > +	}
> > +
> > +	/*
> > +	 * We should normally never reach here, unless min_freq was higher than
> > +	 * the highest available frequency, which is not expected to happen.
> > +	 */
>
> Maybe add a WARN_ONCE(1, "foobar"); here to indicate this unlikely event
> (CPUfreq and EM framwork out of sync)?

Will do.

> [...]