Re: [PATCH 1/4] sched/topology: Store root domain CPU capacity sum

Dietmar Eggemann
2020-04-14T15:28:11Z

On 14.04.20 14:45, Quentin Perret wrote:
> On Wednesday 08 Apr 2020 at 19:03:57 (+0200), Vincent Guittot wrote:
>> Or we can do the opposite and only use capacity_orig_of()/rq->cpu_capacity_orig.
>>
>> Is there a case where the max cpu capacity changes over time ? So I
>> would prefer to use cpu_capacity_orig which is a field of scheduler
>> instead of always calling an external arch specific function
>
> Note however that using arch_scale_cpu_capacity() would be more
> efficient, especially on non-arm/arm64 systems where it is a
> compile-time constant.

or essentially all ARCHs not defining it.

>
> It's probably a matter of personal taste, but I find rq->cpu_capacity_orig
> superfluous. It wastes space without actually giving you anything no?
> Anybody remembers why it was introduced in the first place?

v3.18 arm providing arch_scale_cpu_capacity()
v4.1 introduction of rq->cpu_capacity_orig
v4.10 arm64 providing arch_scale_cpu_capacity()
...

So it's down to the question of 'minimizing the scheduler calls to an
external arch function' vs 'efficiency'.