Re: [PATCH 2/2] sched/fair: Improve the for loop in select_idle_core()

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Feb 07, 2019 at 04:16:06PM +0530, Viresh Kumar wrote:
> @@ -6081,10 +6082,14 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
>  	for_each_cpu_wrap(core, cpus, target) {
>  		bool idle = true;
>  
> -		for_each_cpu(cpu, cpu_smt_mask(core)) {
> -			cpumask_clear_cpu(cpu, cpus);
> -			if (!available_idle_cpu(cpu))
> +		smt = cpu_smt_mask(core);
> +		cpumask_andnot(cpus, cpus, smt);

So where the previous code was like 1-2 stores, you just added 16.

(assuming 64bit and NR_CPUS=1024)

And we still do the iteration anyway:

> +		for_each_cpu(cpu, smt) {
> +			if (!available_idle_cpu(cpu)) {
>  				idle = false;
> +				break;
> +			}
>  		}

An actual improvement would've been:

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 38d4669aa2ef..2d352d6d15c7 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6082,7 +6082,7 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
 		bool idle = true;
 
 		for_each_cpu(cpu, cpu_smt_mask(core)) {
-			cpumask_clear_cpu(cpu, cpus);
+			__cpumask_clear_cpu(cpu, cpus);
 			if (!available_idle_cpu(cpu))
 				idle = false;
 		}