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

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Message-ID: <20130515160546.GD24004@somewhere>
Date:	Wed, 15 May 2013 18:05:48 +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 Wed, May 15, 2013 at 06:09:15AM +0200, Mike Galbraith wrote:
> On Wed, 2013-05-15 at 02:26 +0200, Frederic Weisbecker wrote: 
> > On Tue, May 14, 2013 at 04:07:20PM +0200, Mike Galbraith wrote:
> > > On Tue, 2013-05-14 at 02:57 +0200, Frederic Weisbecker wrote: 
> > > > 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.
> > > 
> > > So with CONFIG_HAVE_UNSTABLE_SCHED_CLOCK, you can't mix sched_clock()
> > > (pure tsc) with local_clock()/sched_clock_cpu(cpu).  The former is
> > > always quite a bit ahead of the later, so mixing clocks is a nogo on
> > > crusty old (but beloved) core2 box.
> > 
> > Right I have the same issue. So let's use local_clock() everywhere here,
> > it takes care of unstable tsc.
> > 
> > Does the following fix the issue for you?
> 
> Yeah, both can use sched_clock_cpu() instead though.

Right, given that irqs are already disabled. I'm preparing the patch.

Thanks!
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