linux-kernel - Re: [PATCH v4 2/2] sched: consider missed ticks in full NOHZ

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Message-ID: <20151102161016.GS3604@twins.programming.kicks-ass.net>
Date:	Mon, 2 Nov 2015 17:10:16 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	byungchul.park@....com
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org, fweisbec@...il.com,
	tglx@...utronix.de
Subject: Re: [PATCH v4 2/2] sched: consider missed ticks in full NOHZ

On Wed, Oct 14, 2015 at 06:47:36PM +0900, byungchul.park@....com wrote:
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4428,7 +4428,7 @@ static void update_idle_cpu_load(struct rq *this_rq)

So if one were to read the comment above update_idle_cpu_load() one
would find there's a problem with jiffy based accounting.

>  /*
>   * Called from tick_nohz_idle_exit() -- try and fix up the ticks we missed.
>   */
> -void update_cpu_load_nohz(void)
> +void update_cpu_load_nohz(int active)

> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 7c7ec45..515edf3 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c

> -static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now)
> +static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now, int active)
>  {
>  	/* Update jiffies first */
>  	tick_do_update_jiffies64(now);
> -	update_cpu_load_nohz();
> +	update_cpu_load_nohz(active);
>  
>  	calc_load_exit_idle();
>  	touch_softlockup_watchdog();

And we could solve all that nicely if we pull up the hrtimer_forward()
result from tick_nohz_restart(), that way we have the actual number of
ticks lost on this cpu, and no need to start guessing about it.

Hmm?
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