linux-kernel - Re: [RFC PATCH v3 1/6] PM: Introduce em_pd_get_higher

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Date:   Thu, 17 Oct 2019 12:09:37 +0100
From:   Douglas Raillard <douglas.raillard@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel@...r.kernel.org
Cc:     linux-pm@...r.kernel.org, mingo@...hat.com, peterz@...radead.org,
        rjw@...ysocki.net, viresh.kumar@...aro.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, qperret@...rret.net,
        patrick.bellasi@...bug.net, dh.han@...sung.com
Subject: Re: [RFC PATCH v3 1/6] PM: Introduce em_pd_get_higher_freq()

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.

> [...]
>