Re: [RFC PATCH 5/6] sched/fair: Select an energy-efficient CPU on task wake-up

Joel Fernandes
2018-03-24T06:06:11Z

On March 23, 2018 6:34:22 PM PDT, Quentin Perret <quentin.perret@arm.com> wrote:
>On Friday 23 Mar 2018 at 18:13:56 (-0700), Joel Fernandes wrote:
>> Hi Morten,
>>
>> On Fri, Mar 23, 2018 at 8:47 AM, Morten Rasmussen
>> <morten.rasmussen@arm.com> wrote:
>> > On Thu, Mar 22, 2018 at 01:10:22PM -0700, Joel Fernandes wrote:
>
>[...]
>
>> > You mean if SD_BALANCE_WAKE isn't set on sched_domains?
>>
>> Yes.
>>
>> > The current code seems to rely on that flag to be set to work
>correctly.
>> > Otherwise, the loop might bail out on !want_affine and we end up
>doing
>> > the find_energy_efficient_cpu() on the lowest level sched_domain
>even if
>> > there is higher level one which isn't over-utilized.
>> >
>> > However, SD_BALANCE_WAKE should be set if SD_ASYM_CPUCAPACITY is
>set so
>> > sd == NULL shouldn't be possible? This only holds as long as we
>only
>> > want EAS for asymmetric systems.
>>
>> Yes, I see you had topology code that set SD_BALANCE_WAKE for ASYM.
>It
>> makes sense to me then, thanks for the clarification.
>>
>> Still I feel it is a bit tedious/confusing when reading code to draw
>> the conclusion about why sd is checked first before doing
>> find_energy_efficient_cpu (and that sd will != NULL for ASYM
>systems).
>> If energy_sd is set, then we can just proceed with EAS without
>> checking that sd != NULL. This function in mainline is already pretty
>> confusing as it is :-(
>
>Right I see your point. The code is correct as is, but I agree that
>having
>a code structured as
>
> if (energy_sd) {
> new_cpu = find_energy_efficient_cpu(energy_sd, p, prev_cpu);
> } else if (!sd) {
> ...
>
>might be easier to understand and functionally equivalent. What do you
>think ?

Yeah definitely. Go for it.

- Joel

--
Sent from my Android device with K-9 Mail. Please excuse my brevity.