Re: [RFC PATCH v2 5/6] sched/fair: Select an energy-efficient CPU on task wake-up

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Apr 06, 2018 at 04:36:06PM +0100, Dietmar Eggemann wrote:
> +	for_each_freq_domain(fd) {
> +		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(fd), sched_domain_span(sd)) {
> +			if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
> +				continue;
> +
> +			if (cpu == prev_cpu)
> +				continue;
> +
> +			util = cpu_util_wake(cpu, p);
> +			cpu_cap = capacity_of(cpu);
> +			if (!util_fits_capacity(util + task_util, cpu_cap))
> +				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;
> +			}
> +		}
> +	}

If each CPU has its own frequency domain, then the above loop ends up
being O(n^2), no? Is there really nothing we can do about that? Also, I
feel that warrants a comment warning about this.

Someone, somewhere will try and build a 64+64 cpu system and get
surprised it doesn't work :-)