linux-kernel - Re: dynticks: CONFIG_VIRT_CPU_ACCOUNTING + CONFIG_CONTEXT_TRACKING breaks accounting on core2 CPUs only

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Message-ID: <20130514005744.GA12749@somewhere>
Date:	Tue, 14 May 2013 02:57:46 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Mike Galbraith <bitbucket@...ine.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: dynticks: CONFIG_VIRT_CPU_ACCOUNTING +  CONFIG_CONTEXT_TRACKING
 breaks accounting on core2 CPUs only

On Sun, May 12, 2013 at 10:17:49AM +0200, Mike Galbraith wrote:
> Greetings,
> 
> Turning on new NO_HZ feature on my Q6600 box in master, I see that tasks
> accrue zero utime/stime.  However, the same exact kernel on E5620 box
> works fine, so it would appear there's a CPU dependency somewhere.

Ah indeed, I just managed to reproduce the same issue.

> 
> Is core2 expected to go dysfunctional with context tracking enabled?
> CONFIG_VIRT_CPU_ACCOUNTING alone works fine in 3.9-stable, turn on
> CONFIG_CONTEXT_TRACKING_FORCE, and CPU accounting stops working on core2
> boxen only, same exact kernel continues to work just fine on E5620
> (Westmere) box.

There was no known issue with core2. The box where I'm seeing the it
is a Phenom quad core that had NR_CPUS=2. May be the issue is more
likely to happen with this low number. I don't know.

I'm investigating further.

Thanks.
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