Re: [PATCH v5 12/14] sched/fair: Select an energy-efficient CPU on task wake-up

[Peter Zijlstra]

On Tue, Jul 24, 2018 at 01:25:19PM +0100, Quentin Perret wrote:
> @@ -6385,18 +6492,26 @@ static int
>  select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_flags)
>  {
>  	struct sched_domain *tmp, *sd = NULL;
> +	struct freq_domain *fd;
>  	int cpu = smp_processor_id();
>  	int new_cpu = prev_cpu;
> -	int want_affine = 0;
> +	int want_affine = 0, want_energy = 0;
>  	int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING);
>  
> +	rcu_read_lock();
>  	if (sd_flag & SD_BALANCE_WAKE) {
>  		record_wakee(p);
> -		want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu)
> -			      && cpumask_test_cpu(cpu, &p->cpus_allowed);
> +		fd = rd_freq_domain(cpu_rq(cpu)->rd);
> +		want_energy = fd && !READ_ONCE(cpu_rq(cpu)->rd->overutilized);
> +		want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu) &&
> +			      cpumask_test_cpu(cpu, &p->cpus_allowed);
> +	}
> +
> +	if (want_energy) {
> +		new_cpu = find_energy_efficient_cpu(p, prev_cpu, fd);
> +		goto unlock;
>  	}
>  

And I suppose you rely on the compiler to optimize that for the static
key inside rd_freq_domain()... Does it do a good job of that?

That is, would not something like:


	rcu_read_lock();
	if (sd_flag & SD_BALANCE_WAKE) {
		record_wakee(p);

		if (static_branch_unlikely(&sched_energy_present)) {
			struct root_domain *rd = cpu_rq(cpu)->rd;
			struct freq_domain *fd = rd_freq_domain(rd);

			if (fd && !READ_ONCE(rd->overutilized)) {
				new_cpu = find_energy_efficient_cpu(p, prev_cpu, fd);
				goto unlock;
			}
		}

		/* ... */
	}


Be far more clear ?