linux-kernel - Re: [PATCH v5 15/23] PM: EM: Optimize em_cpu

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Message-ID: <2a8aa860-17dc-442a-a4ed-8f7c387b15ba@arm.com>
Date: Wed, 10 Jan 2024 13:53:40 +0000
From: Lukasz Luba <lukasz.luba@....com>
To: Qais Yousef <qyousef@...alina.io>
Cc: linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
 rafael@...nel.org, dietmar.eggemann@....com, rui.zhang@...el.com,
 amit.kucheria@...durent.com, amit.kachhap@...il.com,
 daniel.lezcano@...aro.org, viresh.kumar@...aro.org, len.brown@...el.com,
 pavel@....cz, mhiramat@...nel.org, wvw@...gle.com
Subject: Re: [PATCH v5 15/23] PM: EM: Optimize em_cpu_energy() and remove
 division

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.