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

Joel Fernandes
2018-03-25T00:12:20Z

On Fri, Mar 23, 2018 at 6:47 PM, Quentin Perret <quentin.perret@arm.com> wrote:
> On Thursday 22 Mar 2018 at 13:19:03 (-0700), Joel Fernandes wrote:
>> On Thu, Mar 22, 2018 at 11:06 AM, Patrick Bellasi
>> <patrick.bellasi@arm.com> wrote:
>
> [...]
>
>> >> > @@ -6555,6 +6613,14 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
>> >> > break;
>> >> > }
>> >> >
>> >> > + /*
>> >> > + * Energy-aware task placement is performed on the highest
>> >> > + * non-overutilized domain spanning over cpu and prev_cpu.
>> >> > + */
>> >> > + if (want_energy && !sd_overutilized(tmp) &&
>> >> > + cpumask_test_cpu(prev_cpu, sched_domain_span(tmp)))
>> >>
>> >> Shouldn't you check for the SD_ASYM_CPUCAPACITY flag here for tmp level?
>> >
>> > ... and this then should be covered by the previous check in
>> > wake_energy(), which sets want_energy.
>>
>> Right, but in a scenario which probably doesn't exist today where we
>> have both SD_ASYM_CPUCAPACITY and !SD_ASYM_CPUCAPACITY domains in the
>> hierarchy for which want_energy = 1, I was thinking if its more future
>> proof to check it and not make assumptions...
>
> So we can definitely have cases where SD_ASYM_CPUCAPACITY is not set at all
> sd levels. Today, on mobile systems, this flag is typically set only at DIE
> level for big.LITTLE platforms, and not at MC level.
> We enable EAS if we find _at least_ one domain that has this flag in the
> hierarchy, just to make sure we don't enable EAS for symmetric platform.
> It's just a way to check a property about the topology when EAS starts, not
> really a way to actually select the sd at which we do scheduling at
> runtime.

Yes Ok you're right we do have the ASYM flag set at some sd levels but
not others at the moment. Sorry about the hasty comment. I understand
what you're doing now, I am Ok with that.

thanks,

- Joel