linux-kernel - Re: [PATCH 4/4] nohz: Fix iowait overcounting if iowait task migrates

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Message-ID: <20140424191856.GD26782@laptop.programming.kicks-ass.net>
Date:	Thu, 24 Apr 2014 21:18:56 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Fernando Luis Vazquez Cao <fernando_b1@....ntt.co.jp>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 4/4] nohz: Fix iowait overcounting if iowait task migrates

On Thu, Apr 24, 2014 at 08:45:58PM +0200, Denys Vlasenko wrote:
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 268a45e..ffea757 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4218,7 +4218,14 @@ void __sched io_schedule(void)
>  	current->in_iowait = 1;
>  	schedule();
>  	current->in_iowait = 0;
> +#ifdef CONFIG_NO_HZ_COMMON
> +	if (atomic_dec_and_test(&rq->nr_iowait)) {
> +		if (raw_smp_processor_id() != cpu_of(rq))
> +			tick_nohz_iowait_to_idle(cpu_of(rq));
> +	}
> +#else
>  	atomic_dec(&rq->nr_iowait);
> +#endif
>  	delayacct_blkio_end();
>  }

You're really refusing to collapse that stuff eh? Maybe I should just
redirect this entire iowait fest to /dev/null :/

> +void tick_nohz_iowait_to_idle(int cpu)
> +{
> +	struct tick_sched *ts = tick_get_tick_sched(cpu);
> +	ktime_t now = ktime_get();
> +
> +	write_seqcount_begin(&ts->idle_sleeptime_seq);
> +	ts->iowait_exittime = now;
> +	write_seqcount_end(&ts->idle_sleeptime_seq);
> +}


So what again was wrong with this one?

  http://marc.info/?l=linux-kernel&m=139772917211023

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