linux-kernel - Re: [PATCH 1/2] sched/fair: Take thermal pressure into account while estimating energy

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Message-ID: <ce1751d6-4b23-3588-a050-2c30cd3ec9dc@arm.com>
Date:   Wed, 2 Jun 2021 16:35:25 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Quentin Perret <qperret@...gle.com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        peterz@...radead.org, rjw@...ysocki.net, viresh.kumar@...aro.org,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        vincent.donnefort@....com, mingo@...hat.com, juri.lelli@...hat.com,
        rostedt@...dmis.org, segall@...gle.com, mgorman@...e.de,
        bristot@...hat.com
Subject: Re: [PATCH 1/2] sched/fair: Take thermal pressure into account while
 estimating energy

Hi Quentin,

On 6/2/21 4:00 PM, Quentin Perret wrote:
> Hi Lukasz,
> 
> On Wednesday 02 Jun 2021 at 14:56:08 (+0100), Lukasz Luba wrote:
>>   compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
>>   {
>>   	struct cpumask *pd_mask = perf_domain_span(pd);
>> -	unsigned long cpu_cap = arch_scale_cpu_capacity(cpumask_first(pd_mask));
>> +	unsigned long _cpu_cap = arch_scale_cpu_capacity(cpumask_first(pd_mask));
>>   	unsigned long max_util = 0, sum_util = 0;
>> +	unsigned long cpu_cap = _cpu_cap;
>>   	int cpu;
>>   
>>   	/*
>> @@ -6558,6 +6559,14 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
>>   				cpu_util_next(cpu, p, -1) + task_util_est(p);
>>   		}
>>   
>> +		/*
>> +		 * Take the thermal pressure from non-idle CPUs. They have
>> +		 * most up-to-date information. For idle CPUs thermal pressure
>> +		 * signal is not updated so often.
>> +		 */
>> +		if (!idle_cpu(cpu))
>> +			cpu_cap = _cpu_cap - thermal_load_avg(cpu_rq(cpu));
> 
> This messes up the irq time scaling no? Maybe move the capping in this

You are talking about scale_irq_capacity() which shrinks the util by
some percentage of irq time. It might be different, by some fraction
(e.g. 8/9 vs 9/10) compared to SchedUtil view, which passes 'raw' arch
capacity. It then adds the irq part, but still to this slightly
different base util.

> function instead of relying on effective_cpu_util() to do it for you?

Agree, since it would be more 'aligned' with how SchedUtil calls
effective_cpu_util(). I will clamp the returned value.

Thanks for pointing this out.

Regards,
Lukasz