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SubjectRe: [PATCH] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu
On Tue, Nov 23, 2021 at 07:22:29PM +0800, Barry Song wrote:
> From: Barry Song <song.bao.hua@hisilicon.com>
>
> This patch keeps the same scanning amount, but drops a redundant loop
> of cpumask_next_wrap.
> The original code did for_each_cpu_wrap(cpu, cpus, target + 1), then
> checked --nr; this patch does --nr before doing the next loop, thus,
> it can remove a cpumask_next_wrap() which costs a little bit.
>
> Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
> ---
> kernel/sched/fair.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ff69f24..e2fb3e0 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6298,9 +6298,9 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>
> span_avg = sd->span_weight * avg_idle;
> if (span_avg > 4*avg_cost)
> - nr = div_u64(span_avg, avg_cost);
> + nr = div_u64(span_avg, avg_cost) - 1;
> else
> - nr = 4;
> + nr = 3;
>
> time = cpu_clock(this);
> }
> @@ -6312,11 +6312,11 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> return i;
>
> } else {
> - if (!--nr)
> - return -1;
> idle_cpu = __select_idle_cpu(cpu, p);
> if ((unsigned int)idle_cpu < nr_cpumask_bits)
> break;
> + if (!--nr)
> + return -1;
> }
> }

That's just confusing code. Isn't it much clearer to write the whole
thing like so ?

nr--;
for_each_cpu_wrap(cpu, cpus, target+1) {
...
if (!nr--)
return -1;
}

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