Re: [PATCH 1/2] sched/fair: Check if prev_cpu has highest spare cap in feec()

[Pierre Gondois <pierre.gondois@arm.com>]

Hello Peter,

The second patch:
  -[PATCH 2/2] sched/fair: Use IRQ scaling for all sched classes
must be dropped, cf. Vincent Guittot's review, but I believe this patch
should be ok to take if there is no other comment,

Regards,
Pierre

On 8/29/22 07:13, Dietmar Eggemann wrote:
> On 19/08/2022 17:33, Pierre Gondois wrote:
>> When evaluating the CPU candidates in the perf domain (pd) containing
>> the previously used CPU (prev_cpu), find_energy_efficient_cpu()
>> evaluates the energy of the pd:
>> - without the task (base_energy)
>> - with the task placed on prev_cpu (if the task fits)
>> - with the task placed on the CPU with the highest spare capacity,
>>    prev_cpu being excluded from this set
>>
>> If prev_cpu is already the CPU with the highest spare capacity,
>> max_spare_cap_cpu will be the CPU with the second highest spare
>> capacity.
>>
>> On an Arm64 Juno-r2, with a workload of 10 tasks at a 10% duty cycle,
>> when prev_cpu and max_spare_cap_cpu are both valid candidates,
>> prev_spare_cap > max_spare_cap at ~82%.
>> Thus the energy of the pd when placing the task on max_spare_cap_cpu
>> is computed with no possible positive outcome 82% most of the time.
>>
>> Do not consider max_spare_cap_cpu as a valid candidate if
>> prev_spare_cap > max_spare_cap.
>>
>> Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
> 
> LGTM. When I ran the workload I see this happening in 50%-90% of the EAS
> wakeups. This should prevent one needless compute_energy() call out of 7
> on a typical 3-gear system like 2x2x4 in these cases.
> 
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> 
> [...]