linux-kernel - Re: [PATCH 5/5] powercap/drivers/dtpm: Scale the power with the load

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Date:   Tue, 9 Mar 2021 20:03:50 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     rafael@...nel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH 5/5] powercap/drivers/dtpm: Scale the power with the load


Hi Lukasz,

thanks for your comments, one question below.

On 09/03/2021 11:01, Lukasz Luba wrote:

[ ... ]

>>   +static u64 scale_pd_power_uw(struct cpumask *cpus, u64 power)
> 
> renamed 'cpus' into 'pd_mask', see below
> 
>> +{
>> +    unsigned long max, util;
>> +    int cpu, load = 0;
> 
> IMHO 'int load' looks odd when used with 'util' and 'max'.
> I would put in the line above to have them all the same type and
> renamed to 'sum_util'.
> 
>> +
>> +    for_each_cpu(cpu, cpus) {
> 
> I would avoid the temporary CPU mask in the get_pd_power_uw()
> with this modified loop:
> 
> for_each_cpu_and(cpu, pd_mask, cpu_online_mask) {
> 
> 
>> +        max = arch_scale_cpu_capacity(cpu);
>> +        util = sched_cpu_util(cpu, max);
>> +        load += ((util * 100) / max);
> 
> Below you can find 3 optimizations. Since we are not in the hot
> path here, it's up to if you would like to use all/some of them
> or just ignore.
> 
> 1st optimization.
> If we use 'load += (util << 10) / max' in the loop, then
> we could avoid div by 100 and use a right shift:
> (power * load) >> 10
> 
> 2nd optimization.
> Since we use EM CPU mask, which span all CPUs with the same
> arch_scale_cpu_capacity(), you can avoid N divs inside the loop
> and do it once, below the loop.
> 
> 3rd optimization.
> If we just simply add all 'util' into 'sum_util' (no mul or div in
> the loop), then we might just have simple macro
> 
> #define CALC_POWER_USAGE(power, sum_util, max) \
>     (((power * (sum_util << 10)) / max) >> 10)

I don't understand the 'max' division, I was expecting here something
like: ((sum_util << 10) / sum_max) >> 10)

no ?




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