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SubjectRe: [PATCH] sched/fair: Optimize select_idle_cpu
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On 2019/12/12 23:24, Peter Zijlstra wrote:
> On Thu, Dec 12, 2019 at 10:41:02PM +0800, Cheng Jian wrote:
>
>> Fixes: 1ad3aaf3fcd2 ("sched/core: Implement new approach to scale select_idle_cpu()")
> The 'funny' thing is that select_idle_core() actually does the right
> thing.
>
> Copying that should work:
>
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 08a233e97a01..416d574dcebf 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5828,6 +5837,7 @@ static inline int select_idle_smt(struct task_struct *p, int target)
> */
> static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int target)
> {
> + struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
> struct sched_domain *this_sd;
> u64 avg_cost, avg_idle;
> u64 time, cost;
> @@ -5859,11 +5869,11 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>
> time = cpu_clock(this);
>
> - for_each_cpu_wrap(cpu, sched_domain_span(sd), target) {
> + cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> +
> + for_each_cpu_wrap(cpu, cpus, target) {
> if (!--nr)
> return si_cpu;
> - if (!cpumask_test_cpu(cpu, p->cpus_ptr))
> - continue;
> if (available_idle_cpu(cpu))
> break;
> if (si_cpu == -1 && sched_idle_cpu(cpu))
>
> .


in select_idle_smt()

/*
 * Scan the local SMT mask for idle CPUs.
 */
static int select_idle_smt(struct task_struct *p, int target)
{
    int cpu, si_cpu = -1;

    if (!static_branch_likely(&sched_smt_present))
        return -1;

    for_each_cpu(cpu, cpu_smt_mask(target)) {
        if (!cpumask_test_cpu(cpu, p->cpus_ptr))
            continue;
        if (available_idle_cpu(cpu))
            return cpu;
        if (si_cpu == -1 && sched_idle_cpu(cpu))
            si_cpu = cpu;
    }

    return si_cpu;
}


Why don't we do the same thing in this function,

although cpu_smt_present () often has few CPUs.

it is better to determine the 'p->cpus_ptr' first.





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