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From:   Wanpeng Li <kernellwp@gmail.com>
Date:   Mon, 20 Jan 2020 14:14:46 +0800
Message-ID: <CANRm+CwHP63YJ_nbVaNOZhPWNYNwHSTjT3j7aiVnx1pVEf3K9A@mail.gmail.com>
Subject: Re: [PATCH RESEND v2] sched/nohz: Optimize get_nohz_timer_target()
To:     LKML <linux-kernel@vger.kernel.org>
Cc:     Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>
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ping Thomas,
On Mon, 13 Jan 2020 at 08:50, Wanpeng Li <kernellwp@gmail.com> wrote:
>
> From: Wanpeng Li <wanpengli@tencent.com>
>
> On a machine, cpu 0 is used for housekeeping, the other 39 cpus in the
> same socket are in nohz_full mode. We can observe huge time burn in the
> loop for seaching nearest busy housekeeper cpu by ftrace.
>
>   2)               |                        get_nohz_timer_target() {
>   2)   0.240 us    |                          housekeeping_test_cpu();
>   2)   0.458 us    |                          housekeeping_test_cpu();
>
>   ...
>
>   2)   0.292 us    |                          housekeeping_test_cpu();
>   2)   0.240 us    |                          housekeeping_test_cpu();
>   2)   0.227 us    |                          housekeeping_any_cpu();
>   2) + 43.460 us   |                        }
>
> This patch optimizes the searching logic by finding a nearest housekeeper
> cpu in the housekeeping cpumask, it can minimize the worst searching time
> from ~44us to < 10us in my testing. In addition, the last iterated busy
> housekeeper can become a random candidate while current CPU is a better
> fallback if it is a housekeeper.
>
> Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
> v1 -> v2:
>  * current CPU is a better fallback if it is a housekeeper
>
>  kernel/sched/core.c | 19 ++++++++++++-------
>  1 file changed, 12 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 102dfcf..04a0f6a 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -539,27 +539,32 @@ void resched_cpu(int cpu)
>   */
>  int get_nohz_timer_target(void)
>  {
> -       int i, cpu = smp_processor_id();
> +       int i, cpu = smp_processor_id(), default_cpu = -1;
>         struct sched_domain *sd;
>
> -       if (!idle_cpu(cpu) && housekeeping_cpu(cpu, HK_FLAG_TIMER))
> -               return cpu;
> +       if (housekeeping_cpu(cpu, HK_FLAG_TIMER)) {
> +               if (!idle_cpu(cpu))
> +                       return cpu;
> +               default_cpu = cpu;
> +       }
>
>         rcu_read_lock();
>         for_each_domain(cpu, sd) {
> -               for_each_cpu(i, sched_domain_span(sd)) {
> +               for_each_cpu_and(i, sched_domain_span(sd),
> +                       housekeeping_cpumask(HK_FLAG_TIMER)) {
>                         if (cpu == i)
>                                 continue;
>
> -                       if (!idle_cpu(i) && housekeeping_cpu(i, HK_FLAG_TIMER)) {
> +                       if (!idle_cpu(i)) {
>                                 cpu = i;
>                                 goto unlock;
>                         }
>                 }
>         }
>
> -       if (!housekeeping_cpu(cpu, HK_FLAG_TIMER))
> -               cpu = housekeeping_any_cpu(HK_FLAG_TIMER);
> +       if (default_cpu == -1)
> +               default_cpu = housekeeping_any_cpu(HK_FLAG_TIMER);
> +       cpu = default_cpu;
>  unlock:
>         rcu_read_unlock();
>         return cpu;
> --
> 1.8.3.1
>