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

[Mike Galbraith <bitbucket@online.de>]

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.

-Mike