linux-kernel - Re: [PATCH] tick-sched: Update nohz load even if tick already stopped

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Message-ID: <20191009154528.GA18287@lenoir>
Date:   Wed, 9 Oct 2019 17:45:32 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Scott Wood <swood@...hat.com>
Cc:     Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tick-sched: Update nohz load even if tick already stopped

On Wed, Oct 02, 2019 at 06:55:35PM -0400, Scott Wood wrote:
> The way loadavg is tracked during nohz only pays attention to the load
> upon entering nohz.  This can be particularly noticeable if nohz is
> entered while non-idle, and then the cpu goes idle and stays that way for
> a long time.  We've had reports of a loadavg near 150 on a mostly idle
> system.
> 
> Calling calc_load_nohz_start() regardless of whether the tick is already
> stopped addresses the issue when going idle.  Tracking load changes when
> not going idle (e.g. multiple SCHED_FIFO tasks coming and going) is not
> addressed by this patch.
> 
> Signed-off-by: Scott Wood <swood@...hat.com>
> ---
>  kernel/time/tick-sched.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 955851748dc3..f177d8168400 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -763,6 +763,9 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
>  		ts->do_timer_last = 0;
>  	}
>  
> +	/* Even if the tick was already stopped, load may have changed */
> +	calc_load_nohz_start();
> +
>  	/* Skip reprogram of event if its not changed */
>  	if (ts->tick_stopped && (expires == ts->next_tick)) {
>  		/* Sanity check: make sure clockevent is actually programmed */
> @@ -783,7 +786,6 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
>  	 * the scheduler tick in nohz_restart_sched_tick.
>  	 */
>  	if (!ts->tick_stopped) {
> -		calc_load_nohz_start();
>  		quiet_vmstat();
>  
>  		ts->last_tick = hrtimer_get_expires(&ts->sched_timer);


Thanks. I've pondered over your patch to try to avoid calling
calc_load_nohz_start() unconditionally like that. But in the end the
fast path of this function shouldn't bring much overhead and does pretty
much the same as what I would do to call it conditionally.

So I'm applying it.