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

[Peter Zijlstra <peterz@infradead.org>]

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