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

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Message-ID: <d9bea2d0-3869-4f08-8eb8-0ca33ce525ea@arm.com>
Date: Tue, 2 Jan 2024 11:47:02 +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 12/28/23 18:06, Qais Yousef wrote:
> On 11/29/23 11:08, Lukasz Luba wrote:
> 
>> @@ -220,8 +218,9 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
>>   				return -EINVAL;
>>   			}
>>   		} else {
>> -			power_res = table[i].power;
>> -			cost = div64_u64(fmax * power_res, table[i].frequency);
>> +			/* increase resolution of 'cost' precision */
>> +			power_res = table[i].power * 10;
> 
> Power is in uW, right? You're just taking advantage here that everything will
> use this new cost field so you can add as many 0s to improve resolution without
> impact elsewhere that care to compare using the same units?

This code doesn't overwrite the 'power' field value. The 'cost' value is
only used in EAS, so yes I just want to increase resolution there.

I think you mixed 'power' and 'cost' fields. We don't compare 'cost'
anywhere. We just use 'cost' in one place em_cpu_energy() and we
multiply it (not compare it).

> 
> Did you see a problem or just being extra cautious here?

There is no problem, 'cost' is a private coefficient for EAS only.

> 
>> +			cost = power_res / table[i].performance;
>>   		}
>>   
>>   		table[i].cost = cost;
>> -- 
>> 2.25.1
>>