linux-kernel - Re: NOHZ: WARNING: at arch/x86/kernel/smp.c:123 native_smp_send

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Date:	Tue, 21 May 2013 09:21:40 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Michael Wang <wangyun@...ux.vnet.ibm.com>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>, Tejun Heo <tj@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Tony Luck <tony.luck@...el.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>, rjw@...k.pl,
	cpufreq@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: NOHZ: WARNING: at arch/x86/kernel/smp.c:123
 native_smp_send_reschedule, round 2

On Tue, May 21, 2013 at 10:20:51AM +0800, Michael Wang wrote:
> This is not enough to prove that policy->cpus is wrong, the cpu could
> be online when get from policy->cpus, but offline when checked here,
> since hotplug is able to happen during the period.

Strictly speaking you're correct but I don't do any hotplug besides the
one-time thing which is part of halting the box.

> I don't get it...
> 
> get_online_cpus() is just stop hotplug happen after it was invoked, so
> unless policy->cpus is really wrong, otherwise all the cpu it masked
> won't go offline any more.

Yes, that's my impression too - at the point we do gov_queue_work,
policy->cpus already contains offline cpus.

> This protect nothing...before we go here, the cpu could already
> offline, nothing changed...

Yes, but I don't want to schedule work on an offlined cpu and that is
ensured here.

> If you really want to confirm the policy->cpus was wrong, the way
> should be apply the fix I suggested, than check online in here.

Sure, feel free to get a box, enable NO_HZ_FULL and do all the
experimentations you desire. I surely cannot be the only one who
triggers this.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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