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SubjectRe: [RFC PATCH 1/7] PM: Introduce em_pd_get_higher_freq()
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Hi Patrick,

On 5/16/19 1:42 PM, Patrick Bellasi wrote:
> On 08-May 18:42, douglas.raillard@arm.com wrote:
>> From: Douglas RAILLARD <douglas.raillard@arm.com>
>>
>> em_pd_get_higher_freq() returns a frequency greater or equal to the
>> provided one while taking into account a given cost margin. It also
>> skips inefficient OPPs that have a higher cost than another one with a
>> higher frequency.
>
> It's worth to add a small description and definition of what we mean by
> "OPP efficiency". Despite being just an RFC, it could help to better
> understand what we are after.

Right, here efficiency=capacity/power.

>
> [...]
>
>> +/** + * 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, as a per-1024 value.
> ^^^^^^^^
> here...
>
>> + *
>> + * 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) / 1024;
> ^^^^
> ... end here we should probably better use SCHED_CAPACITY_SCALE
> instead of hard-coding in values, isn't it?

"cs->cost*cost_margin/1024" is not a capacity, it's a cost as defined here:
https://elixir.bootlin.com/linux/latest/source/include/linux/energy_model.h#L17

Actually, it's in milliwatts, but it's not better the better way to look at
it to understand it IMHO.

The margin is expressed as a "per-1024" value just like we use percents'
in everyday life, so it has no unit. If we want to avoid hard-coded values
here, I can introduce an ENERGY_COST_MARGIN_SCALE macro.

>> + break;
>> + }
>> + }
>> +
>
> [...]
>
> Best,
> Patrick

Thanks,
Douglas

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