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