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

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Message-ID: <11976c37-65d3-e0c6-034d-cfec9ebb5b49@arm.com>
Date:   Wed, 19 Jun 2019 17:08:37 +0100
From:   Douglas Raillard <douglas.raillard@....com>
To:     Patrick Bellasi <patrick.bellasi@....com>,
        Quentin Perret <quentin.perret@....com>
Cc:     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()

Hi Patrick,

On 5/16/19 2:22 PM, Patrick Bellasi wrote:
> 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.

Would it be a good idea to use SCHED_FIXEDPOINT_SCALE in energy.c ?
Since it's not part of the scheduler, maybe there is a scale covering a wider scope,
or we can introduce a similar ENERGY_FIXEDPOINT_SCALE in energy_model.h.


>> Thanks,
>> Quentin
> 

Thanks,
Douglas