linux-kernel - Re: [PATCH 1/2] nohz: stop updating sleep stats from get_cpu_{idle,iowait}_time

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Message-ID: <20140415084854.GL11096@twins.programming.kicks-ass.net>
Date:	Tue, 15 Apr 2014 10:48:54 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org,
	Fernando Luis Vazquez Cao <fernando_b1@....ntt.co.jp>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Frederic Weisbecker <fweisbec@...il.com>,
	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>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	stable@...r.kernel.org
Subject: Re: [PATCH 1/2] nohz: stop updating sleep stats from
 get_cpu_{idle,iowait}_time_us()

On Thu, Apr 10, 2014 at 06:11:03PM +0900, Hidetoshi Seto wrote:
> -	if (ts->idle_active) {
> -		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;
> -	}
> -
> -	if (last_update_time)
> -		*last_update_time = ktime_to_us(now);
> -
> -}
> -
> -static void tick_nohz_stop_idle(struct tick_sched *ts, ktime_t now)
> -{
> -	update_ts_time_stats(smp_processor_id(), ts, now, NULL);
> +	/* Updates the per cpu time idle statistics counters */
> +	delta = ktime_sub(now, ts->idle_entrytime);
> +	if (nr_iowait_cpu(smp_processor_id()) > 0)

Now I know the old code also uses nr_iowait_cpu(), but that function is
crackbrained. This cannot possibly be right.

Anything using nr_iowait_cpu() is broken beyond repair.
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