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

[Dietmar Eggemann]

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).

> +			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)?

[...]