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

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Message-ID: <661068a2-7c46-4703-ba4d-5ce1cdf44b3d@arm.com>
Date: Mon, 15 Jan 2024 12:36:30 +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 1/15/24 12:21, Qais Yousef wrote:
> On 01/10/24 13:53, Lukasz Luba wrote:
>>
>>
>> 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.
> 
> So it seems a uW is not sufficient enough. We moved from mW because of
> resolution already. Shall we make it nW then and multiply by 1000 always? The
> choice of 10 looks arbitrary IMHO
> 

No, there is no need of nW in the 'power' field for this.
You've missed the point.