linux-kernel - Re: [PATCH 1/4] nohz: Only update sleeptime stats locally

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Date:	Sun, 18 Aug 2013 18:49:37 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	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>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: [PATCH 1/4] nohz: Only update sleeptime stats locally

On 08/16, Frederic Weisbecker wrote:
>
> To fix this, lets only update the sleeptime stats locally when the CPU
> exits from idle.

I am in no position to ack the changes in this area, but I like this
change very much. Because, as a code reader, I was totally confused by

	if (last_update_time)
		update_ts_time_stats()

code and it looks "obviously wrong".

I added more cc's. It seems to me that 9366d840 "cpufreq: governors:
Calculate iowait time only when necessary" doesn't realize what

	-       u64 idle_time = get_cpu_idle_time_us(cpu, NULL);
	+       u64 idle_time = get_cpu_idle_time_us(cpu, io_busy ? wall : NULL);

actually means. OTOH, get_cpu_iowait_time_us() was called with
last_update_time != NULL even before this patch...

In short. This looks like the clear fix to me, but I do not understand
this code enough, and I think that cpufreq should know about this change.

>  static void tick_nohz_stop_idle(int cpu, ktime_t now)
>  {
>  	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
> +	ktime_t delta;
>  
> -	update_ts_time_stats(cpu, ts, now, NULL);
> +	/* Updates the per cpu time idle statistics counters */
> +	delta = ktime_sub(now, ts->idle_entrytime);
> +	if (nr_iowait_cpu(cpu) > 0)
> +		ts->iowait_sleeptime = ktime_add(ts->iowait_sleeptime, delta);
> +	else
> +		ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
> +	ts->idle_entrytime = now;
>  	ts->idle_active = 0;

With or without this change, why we update ->idle_entrytime in this case?
Looks harmless, but a bit confusing.

While this doesn't really matter, we could probably even kill ->idle_active
and use !!ts->idle_entrytime instead.

> @@ -473,17 +458,14 @@ u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)

And I think that we should kill this last_update_time argument later.

Oleg.

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