Re: [PATCH v5 15/23] PM: EM: Optimize em_cpu_energy() and remove division

Lukasz Luba
2024-01-10T13:52:21Z

On 1/4/24 19:23, Qais Yousef wrote:
> On 01/02/24 11:47, Lukasz Luba wrote:
>>> Did you see a problem or just being extra cautious here?
>>
>> There is no problem, 'cost' is a private coefficient for EAS only.
>
> Let me ask differently, what goes wrong if you don't increase the resolution
> here? Why is it necessary?
>

When you have 800mW at CPU capacity 1024, then the value is small (below
1 thousand).
Example:
power = 800000 uW
cost = 800000 / 1024 = 781

While I know from past that sometimes OPPs might have close voltage
values and a rounding could occur and make some OPPs inefficient
while they aren't.

This is what would happen when we have the 1x resolution:
/sys/kernel/debug/energy_model/cpu4/ps:1008000/cost:551
/sys/kernel/debug/energy_model/cpu4/ps:1200000/cost:644
/sys/kernel/debug/energy_model/cpu4/ps:1416000/cost:744
/sys/kernel/debug/energy_model/cpu4/ps:1512000/cost:851
/sys/kernel/debug/energy_model/cpu4/ps:408000/cost:493
/sys/kernel/debug/energy_model/cpu4/ps:600000/cost:493
/sys/kernel/debug/energy_model/cpu4/ps:816000/cost:493
The bottom 3 OPPs have the same 'cost' thus 2 OPPs are in-efficient,
which is not true (see below).

This is what would happen when we have the 10x resolution:
/sys/kernel/debug/energy_model/cpu4/ps:1008000/cost:5513
/sys/kernel/debug/energy_model/cpu4/ps:1200000/cost:6443
/sys/kernel/debug/energy_model/cpu4/ps:1416000/cost:7447
/sys/kernel/debug/energy_model/cpu4/ps:1512000/cost:8514
/sys/kernel/debug/energy_model/cpu4/ps:408000/cost:4934
/sys/kernel/debug/energy_model/cpu4/ps:600000/cost:4933
/sys/kernel/debug/energy_model/cpu4/ps:816000/cost:4934
Here the OPP with 600MHz is more efficient than 408MHz,
which is true. So only 408MHz will be marked as in-efficient OPP.

This is what would happen when we have the 100x resolution:
/sys/kernel/debug/energy_model/cpu4/ps:1008000/cost:55137
/sys/kernel/debug/energy_model/cpu4/ps:1200000/cost:64433
/sys/kernel/debug/energy_model/cpu4/ps:1416000/cost:74473
/sys/kernel/debug/energy_model/cpu4/ps:1512000/cost:85140
/sys/kernel/debug/energy_model/cpu4/ps:408000/cost:49346
/sys/kernel/debug/energy_model/cpu4/ps:600000/cost:49331
/sys/kernel/debug/energy_model/cpu4/ps:816000/cost:49346
The higher (100x) resolution does not bring that much in
practice.

If you have other questions, let's continue on v6 series.