Re: [RFC PATCH v3 09/10] sched/fair: Select an energy-efficient CPU on task wake-up

[Pavan Kondeti]

On Mon, May 21, 2018 at 03:25:04PM +0100, Quentin Perret wrote:

<snip>

> +	if (cpumask_test_cpu(prev_cpu, &p->cpus_allowed))
> +		prev_energy = best_energy = compute_energy(p, prev_cpu);
> +	else
> +		prev_energy = best_energy = ULONG_MAX;
> +
> +	for_each_freq_domain(sfd) {
> +		unsigned long spare_cap, max_spare_cap = 0;
> +		int max_spare_cap_cpu = -1;
> +		unsigned long util;
> +
> +		/* Find the CPU with the max spare cap in the freq. dom. */
> +		for_each_cpu_and(cpu, freq_domain_span(sfd), sched_domain_span(sd)) {
> +			if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
> +				continue;
> +
> +			if (cpu == prev_cpu)
> +				continue;
> +
> +			/* Skip CPUs that will be overutilized */
> +			util = cpu_util_wake(cpu, p) + task_util;
> +			cpu_cap = capacity_of(cpu);
> +			if (cpu_cap * 1024 < util * capacity_margin)
> +				continue;
> +
> +			spare_cap = cpu_cap - util;
> +			if (spare_cap > max_spare_cap) {
> +				max_spare_cap = spare_cap;
> +				max_spare_cap_cpu = cpu;
> +			}
> +		}
> +
> +		/* Evaluate the energy impact of using this CPU. */
> +		if (max_spare_cap_cpu >= 0) {
> +			cur_energy = compute_energy(p, max_spare_cap_cpu);
> +			if (cur_energy < best_energy) {
> +				best_energy = cur_energy;
> +				best_energy_cpu = max_spare_cap_cpu;
> +			}
> +		}
> +	}
> +
> +	/*
> +	 * We pick the best CPU only if it saves at least 1.5% of the
> +	 * energy used by prev_cpu.
> +	 */
> +	if ((prev_energy - best_energy) > (prev_energy >> 6))
> +		return best_energy_cpu;
> +
> +	return prev_cpu;
> +}

We are comparing the best_energy_cpu against prev_cpu even when prev_cpu
can't accommodate the waking task. Is this intentional? Should not we
discard the prev_cpu if it can't accommodate the task.

This can potentially make a BIG task run on a lower capacity CPU until
load balancer kicks in and corrects the situation.

Thanks,
Pavan
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