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

[Frederic Weisbecker <fweisbec@gmail.com>]

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?

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index cc2dc3e..1ce322f 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -747,7 +748,7 @@ void arch_vtime_task_switch(struct task_struct *prev)
 
 	write_seqlock(&current->vtime_seqlock);
 	current->vtime_snap_whence = VTIME_SYS;
-	current->vtime_snap = sched_clock();
+	current->vtime_snap = local_clock();
 	write_sequnlock(&current->vtime_seqlock);
 }
 
@@ -757,7 +758,7 @@ void vtime_init_idle(struct task_struct *t)
 
 	write_seqlock_irqsave(&t->vtime_seqlock, flags);
 	t->vtime_snap_whence = VTIME_SYS;
-	t->vtime_snap = sched_clock();
+	t->vtime_snap = local_clock();
 	write_sequnlock_irqrestore(&t->vtime_seqlock, flags);
 }