linux-kernel - Re: [PATCH RESEND 0/4] nohz: Fix racy sleeptime stats

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Date:	Wed, 21 Aug 2013 10:28:01 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	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>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: [PATCH RESEND 0/4] nohz: Fix racy sleeptime stats

On Tue, Aug 20, 2013 at 08:15:00PM +0200, Oleg Nesterov wrote:
> While at it.
> 
> I do not also understand the cpu_online() checks in fs/proc/stat.c.
> 
> OK, I agree, if cpu is offline it should not participate in cpu
> summary. But if it goes offline, why it should switch from
> ->iowait_sleeptime + cpustat[CPUTIME_IDLE] as it seen by /proc/stat?
> 
> This can be another source of "idle goes backward", no?
> 
> IOW. Ignoring the other problems we have, perhaps something like
> below makes sense?


Agreed, however

> 
> +++ x/kernel/time/tick-sched.c
> @@ -477,7 +477,7 @@ u64 get_cpu_idle_time_us(int cpu, u64 *l
>  		update_ts_time_stats(cpu, ts, now, last_update_time);
>  		idle = ts->idle_sleeptime;
>  	} else {
> -		if (ts->idle_active && !nr_iowait_cpu(cpu)) {
> +		if (ts->idle_active && cpu_online(cpu) && !nr_iowait_cpu(cpu)) {
>  			ktime_t delta = ktime_sub(now, ts->idle_entrytime);
>  
>  			idle = ktime_add(ts->idle_sleeptime, delta);
> @@ -518,7 +518,7 @@ u64 get_cpu_iowait_time_us(int cpu, u64 
>  		update_ts_time_stats(cpu, ts, now, last_update_time);
>  		iowait = ts->iowait_sleeptime;
>  	} else {
> -		if (ts->idle_active && nr_iowait_cpu(cpu) > 0) {
> +		if (ts->idle_active && cpu_online(cpu) && nr_iowait_cpu(cpu)) {
>  			ktime_t delta = ktime_sub(now, ts->idle_entrytime);
>  
>  			iowait = ktime_add(ts->iowait_sleeptime, delta);
> 

That's still mighty odd, but I guess that's in part due to the whacky
semantics. We could simply transfer any open nr_iowait to the cpu
doing the hotplug and then we have offline cpus that have nr_iowait == 0
and the above becomes simpler again.
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