linux-kernel - Re: [PATCH] sched/nohz: Optimize get nohz timer target

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Date:   Wed, 19 Jun 2019 08:42:19 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH] sched/nohz: Optimize get nohz timer target

Cc Frederic,
On Tue, 18 Jun 2019 at 11:13, Wanpeng Li <kernellwp@...il.com> wrote:
>
> From: Wanpeng Li <wanpengli@...cent.com>
>
> On a machine, cpu 0 is used for housekeeping, other 39 cpus 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.
>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  kernel/sched/core.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 83bd6bb..db550cf 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -548,11 +548,12 @@ int get_nohz_timer_target(void)
>
>         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;
>                         }
> --
> 2.7.4
>