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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Fri, 13 Dec 2019 09:37:27 +0100
Message-ID: <CAKfTPtCQCQio=D3nRTRgbhthKWo752OeaM2X4UcNwr2jByvoNg@mail.gmail.com>
Subject: Re: [PATCH v2] sched/fair: Optimize select_idle_cpu
To:     Cheng Jian <cj.chengjian@huawei.com>
Cc:     Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        chenwandun@huawei.com, Xie XiuQi <xiexiuqi@huawei.com>,
        liwei391@huawei.com, huawei.libin@huawei.com,
        bobo.shaobowang@huawei.com, Juri Lelli <juri.lelli@redhat.com>
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On Fri, 13 Dec 2019 at 03:48, Cheng Jian <cj.chengjian@huawei.com> wrote:
>
> select_idle_cpu() will scan the LLC domain for idle CPUs,
> it's always expensive. so the next commit :
>
>         1ad3aaf3fcd2 ("sched/core: Implement new approach to scale select_idle_cpu()")
>
> introduces a way to limit how many CPUs we scan.
>
> But it consume some CPUs out of 'nr' that are not allowed
> for the task and thus waste our attempts. The function
> always return nr_cpumask_bits, and we can't find a CPU
> which our task is allowed to run.
>
> Cpumask may be too big, similar to select_idle_core(), use
> per_cpu_ptr 'select_idle_mask' to prevent stack overflow.
>
> Fixes: 1ad3aaf3fcd2 ("sched/core: Implement new approach to scale select_idle_cpu()")
> Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>

Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>

> ---
>  kernel/sched/fair.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 08a233e97a01..d48244388ce9 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5828,6 +5828,7 @@ 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 +5860,11 @@ 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))
> --
> 2.20.1
>