Re: [PATCH 2/2] nohz: use delayed iowait accounting to avoid race on idle time stats

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Apr 10, 2014 at 06:13:54PM +0900, Hidetoshi Seto wrote:
> [WHAT THIS PATCH PROPOSED]:
> 
> To fix problem 1, this patch adds seqcount for NO_HZ idle
> accounting to avoid possible races between reader/writer.
> 
> And to cope with problem 2, I introduced delayed iowait
> accounting to get approximate value without making observers
> to writers. Refer comment in patch for the detail.

> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -407,15 +407,42 @@ static void tick_nohz_stop_idle(struct tick_sched *ts, ktime_t now)
>  {
>  	ktime_t delta;
>  
> +	write_seqcount_begin(&ts->idle_sleeptime_seq);
> +
>  	/* Updates the per cpu time idle statistics counters */
>  	delta = ktime_sub(now, ts->idle_entrytime);
> +
> +	/*
> +	 * Perform delayed iowait accounting:
> +	 *
> +	 * We account sleep time as iowait when nr_iowait of cpu indicates
> +	 * there are taskes blocked by io, at the end of idle (=here).
> +	 * It means we can not determine whether the sleep time will be idle
> +	 * or iowait on the fly.
> +	 * Therefore introduce a new rule:
> +	 * - basically observers assign delta to idle
> +	 * - if cpu find nr_iowait>0 at idle exit, accumulate delta as missed
> +	 *   iowait, and account it in next turn of sleep instead.
> +	 * - if observer find accumulated iowait while cpu is in sleep, it
> +	 *   can calculate proper value to be accounted.
> +	 */
> +	if (ktime_compare(ts->iowait_pending, delta) > 0) {
>  		ts->iowait_sleeptime = ktime_add(ts->iowait_sleeptime, delta);
> +		ts->iowait_pending = ktime_sub(ts->iowait_pending, delta);
> +	} else {
> +		ts->idle_sleeptime = ktime_add(ts->idle_sleeptime,
> +					ktime_sub(delta, ts->iowait_pending));
> +		ts->iowait_sleeptime = ktime_add(ts->iowait_sleeptime,
> +						 ts->iowait_pending);
> +		ts->iowait_pending = ktime_set(0, 0);
> +	}
> +	if (nr_iowait_cpu(smp_processor_id()) > 0)
> +		ts->iowait_pending = ktime_add(ts->iowait_pending, delta);
> +
>  	ts->idle_active = 0;
>  
> +	write_seqcount_end(&ts->idle_sleeptime_seq);
> +
>  	sched_clock_idle_wakeup_event(0);
>  }

Why!? Both changelog and comment are silent on this. This doesn't appear
to make any sense nor really solve anything.