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Date:   Tue, 31 May 2022 10:17:01 +0200
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From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
Subject: Re: [PATCH v9 6/7] sched/fair: Remove task_util from effective
 utilization in feec()
To:     Vincent Donnefort <vdonnefort@google.com>, peterz@infradead.org,
        mingo@redhat.com, vincent.guittot@linaro.org
Cc:     linux-kernel@vger.kernel.org, morten.rasmussen@arm.com,
        chris.redpath@arm.com, qperret@google.com, tao.zhou@linux.dev,
        kernel-team@android.com
References: <20220523155140.2878563-1-vdonnefort@google.com>
 <20220523155140.2878563-7-vdonnefort@google.com>
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- Vincent Donnefort <vincent.donnefort@arm.com>

On 23/05/2022 17:51, Vincent Donnefort wrote:
> From: Vincent Donnefort <vincent.donnefort@arm.com>

[...]

> @@ -6834,22 +6887,36 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>  	if (!task_util_est(p))
>  		goto unlock;
>  
> +	eenv_task_busy_time(&eenv, p, prev_cpu);
> +
>  	for (; pd; pd = pd->next) {
> -		unsigned long cur_delta, spare_cap, max_spare_cap = 0;
> +		unsigned long cpu_cap, cpu_thermal_cap, util;
> +		unsigned long cur_delta, max_spare_cap = 0;
>  		bool compute_prev_delta = false;
>  		unsigned long base_energy_pd;
>  		int max_spare_cap_cpu = -1;
>  
>  		cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask);

Internal EAS testing of this patch-set version has revealed that this
doesn't work against LTP CPU hotplug stress test. `struct cpumask *cpus`
can't be used when it is empty. This can happen in case all PD CPUs are
hotplugged out since we `and` the invariant PD cpumask with
cpu_online_mask. We need a:

+               if (cpumask_empty(cpus))
+                       continue;
+

here.

> -		for_each_cpu_and(cpu, cpus, sched_domain_span(sd)) {
> +		/* Account thermal pressure for the energy estimation */
> +		cpu = cpumask_first(cpus);
> +		cpu_thermal_cap = arch_scale_cpu_capacity(cpu);
> +		cpu_thermal_cap -= arch_scale_thermal_pressure(cpu);

[...]