linux-kernel - Re: [PATCH v4] time/tick-sched: idle load balancing when nohz

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Date: Wed, 15 May 2024 17:41:33 +0100
From: Yun Levi <ppbuk5246@...il.com>
To: anna-maria@...utronix.de, frederic@...nel.org, mingo@...nel.org, 
	tglx@...utronix.de, Markus.Elfring@....de
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] time/tick-sched: idle load balancing when nohz_full
 cpu becomes idle.

Gentle ping..?

Am I missing something?

Thanks.

On Thu, May 9, 2024 at 10:29 AM Levi Yun <ppbuk5246@...il.com> wrote:
>
> When nohz_full CPU stops tick in tick_nohz_irq_exit(),
> It wouldn't be chosen to perform idle load balancing because it doesn't
> call nohz_balance_enter_idle() in tick_nohz_idle_stop_tick() when it
> becomes idle.
>
> Formerly, __tick_nohz_idle_enter() is called in both
> tick_nohz_irq_exit() and in do_idle().
> That's why commit a0db971e4eb6 ("nohz: Move idle balancer registration
> to the idle path") prevents nohz_full cpu which isn't yet
> idle state but tick is stopped from entering idle balance.
>
> However, this prevents nohz_full cpu which already stops tick from
> entering idle balacne when this cpu really becomes idle state.
>
> Currently, tick_nohz_idle_stop_tick() is only called in idle state and
> it calls nohz_balance_enter_idle(). this function tracks the CPU
> which is part of nohz.idle_cpus_mask with rq->nohz_tick_stopped properly.
>
> Therefore, Change tick_nohz_idle_stop_tick() to call nohz_balance_enter_idle()
> without checking !was_stopped so that nohz_full cpu can be chosen to
> perform idle load balancing when it enters idle state.
>
> Fixes: a0db971e4eb6 ("nohz: Move idle balancer registration to the idle path")
> Signed-off-by: Levi Yun <ppbuk5246@...il.com>
> ---
> v4:
>         - Add fixes tags.
>
> v3:
>         - Rewording commit message.
>
> v2:
>         - Fix typos in commit message.
>
>  kernel/time/tick-sched.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 71a792cd8936..31a4cd89782f 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -1228,8 +1228,10 @@ void tick_nohz_idle_stop_tick(void)
>                 ts->idle_sleeps++;
>                 ts->idle_expires = expires;
>
> -               if (!was_stopped && tick_sched_flag_test(ts, TS_FLAG_STOPPED)) {
> -                       ts->idle_jiffies = ts->last_jiffies;
> +               if (tick_sched_flag_test(ts, TS_FLAG_STOPPED)) {
> +                       if (!was_stopped)
> +                               ts->idle_jiffies = ts->last_jiffies;
> +
>                         nohz_balance_enter_idle(cpu);
>                 }
>         } else {
> --
> 2.41.0