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Message-ID: <1368517039.5885.3.camel@marge.simpson.net>
Date: Tue, 14 May 2013 09:37:19 +0200
From: Mike Galbraith <bitbucket@...ine.de>
To: Frederic Weisbecker <fweisbec@...il.com>
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 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.
Me too.
bash-6023 [001] d... 290.494214: vtime_delta: clock: 289702961236 vtime_snap: 290493017701
Always. Not good.
I see..
current->vtime_snap = sched_clock();
and..
clock = local_clock();
Things that make ya go hmm. The below "fixes" it (not).
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index cc2dc3ee..3133665 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -634,14 +634,17 @@ void thread_group_cputime_adjusted(struct task_struct *p, cputime_t *ut, cputime
#endif /* !CONFIG_VIRT_CPU_ACCOUNTING_NATIVE */
#ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
-static unsigned long long vtime_delta(struct task_struct *tsk)
+static noinline unsigned long long vtime_delta(struct task_struct *tsk)
{
unsigned long long clock;
- clock = local_clock();
+// clock = local_clock();
+ clock = sched_clock();
+ trace_printk("clock: %Lu vtime_snap: %Lu\n", clock, tsk->vtime_snap);
if (clock < tsk->vtime_snap)
return 0;
+ trace_printk("clock: %Lu vtime_snap: %Lu returns :%Lu\n", clock, tsk->vtime_snap, clock - tsk->vtime_snap);
return clock - tsk->vtime_snap;
}
--
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