linux-kernel - Re: [PATCH v8 2/4] sched: Don't account irq time if sched_clock

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Message-ID: <CAKfTPtBXA87V=ZnKxnyku7LWe1+UOzsnkDwCENNxoC3DyTVqcA@mail.gmail.com>
Date: Fri, 3 Jan 2025 14:32:26 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Yafang Shao <laoar.shao@...il.com>
Cc: peterz@...radead.org, mingo@...hat.com, mkoutny@...e.com, 
	hannes@...xchg.org, juri.lelli@...hat.com, dietmar.eggemann@....com, 
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com, 
	surenb@...gle.com, linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, 
	lkp@...el.com
Subject: Re: [PATCH v8 2/4] sched: Don't account irq time if
 sched_clock_irqtime is disabled

On Fri, 3 Jan 2025 at 03:24, Yafang Shao <laoar.shao@...il.com> wrote:
>
> sched_clock_irqtime may be disabled due to the clock source, in which case
> IRQ time should not be accounted. Let's add a conditional check to avoid
> unnecessary logic.
>
> Signed-off-by: Yafang Shao <laoar.shao@...il.com>
> Reviewed-by: Michal Koutný <mkoutny@...e.com>

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


> ---
>  kernel/sched/core.c | 44 +++++++++++++++++++++++---------------------
>  1 file changed, 23 insertions(+), 21 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 84902936a620..22dfcd3e92ed 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -740,29 +740,31 @@ static void update_rq_clock_task(struct rq *rq, s64 delta)
>         s64 __maybe_unused steal = 0, irq_delta = 0;
>
>  #ifdef CONFIG_IRQ_TIME_ACCOUNTING
> -       irq_delta = irq_time_read(cpu_of(rq)) - rq->prev_irq_time;
> +       if (irqtime_enabled()) {
> +               irq_delta = irq_time_read(cpu_of(rq)) - rq->prev_irq_time;
>
> -       /*
> -        * Since irq_time is only updated on {soft,}irq_exit, we might run into
> -        * this case when a previous update_rq_clock() happened inside a
> -        * {soft,}IRQ region.
> -        *
> -        * When this happens, we stop ->clock_task and only update the
> -        * prev_irq_time stamp to account for the part that fit, so that a next
> -        * update will consume the rest. This ensures ->clock_task is
> -        * monotonic.
> -        *
> -        * It does however cause some slight miss-attribution of {soft,}IRQ
> -        * time, a more accurate solution would be to update the irq_time using
> -        * the current rq->clock timestamp, except that would require using
> -        * atomic ops.
> -        */
> -       if (irq_delta > delta)
> -               irq_delta = delta;
> +               /*
> +                * Since irq_time is only updated on {soft,}irq_exit, we might run into
> +                * this case when a previous update_rq_clock() happened inside a
> +                * {soft,}IRQ region.
> +                *
> +                * When this happens, we stop ->clock_task and only update the
> +                * prev_irq_time stamp to account for the part that fit, so that a next
> +                * update will consume the rest. This ensures ->clock_task is
> +                * monotonic.
> +                *
> +                * It does however cause some slight miss-attribution of {soft,}IRQ
> +                * time, a more accurate solution would be to update the irq_time using
> +                * the current rq->clock timestamp, except that would require using
> +                * atomic ops.
> +                */
> +               if (irq_delta > delta)
> +                       irq_delta = delta;
>
> -       rq->prev_irq_time += irq_delta;
> -       delta -= irq_delta;
> -       delayacct_irq(rq->curr, irq_delta);
> +               rq->prev_irq_time += irq_delta;
> +               delta -= irq_delta;
> +               delayacct_irq(rq->curr, irq_delta);
> +       }
>  #endif
>  #ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING
>         if (static_key_false((&paravirt_steal_rq_enabled))) {
> --
> 2.43.5
>