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Date: Wed, 13 Nov 2013 11:23:38 -0500
From: Steven Rostedt <rostedt@goodmis.org>
To: paulmck@linux.vnet.ibm.com
Cc: Thomas Gleixner <tglx@linutronix.de>,
        Matthew Whitehead <tedheadster@gmail.com>, john.stultz@linaro.org,
        linux-kernel@vger.kernel.org, mwhitehe@redhat.com
Subject: Re: nohz problem with idle time on old hardware
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On Wed, 13 Nov 2013 08:18:29 -0800
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> On Wed, Nov 13, 2013 at 11:12:57AM -0500, Steven Rostedt wrote:
> > On Wed, 13 Nov 2013 17:07:18 +0100 (CET)
> > Thomas Gleixner <tglx@linutronix.de> wrote:
> > 
> > 
> > > Right. It's telling you if NOHZ is enabled. It's not telling you that
> > > NOHZ is active.
> > 
> > Yeah, which makes this code rather silly:
> > 
> > in rcu_prepare_for_idle():
> > 
> > 	/* Handle nohz enablement switches conservatively. */
> > 	tne = ACCESS_ONCE(tick_nohz_enabled);
> > 	if (tne != rdtp->tick_nohz_enabled_snap) {
> > 		if (rcu_cpu_has_callbacks(cpu, NULL))
> > 			invoke_rcu_core(); /* force nohz to see update. */
> > 		rdtp->tick_nohz_enabled_snap = tne;
> > 		return;
> > 	}
> 
> OK, what should I be checking instead?  Not much point in trying to
> get RCU out of the way of disabling the scheduling-clock interrupt
> if NOHZ is disabled.  ;-)
> 

I'll leave the answer to Thomas, but checking tick_nohz_enabled just
lets you know if someone booted with nohz=off or not (and has nohz
configured). But it doesn't tell you if nohz is actually being used.

That is, tick_nohz_enabled is set at bootup and never changes.

Perhaps this old hardware uncovered other bugs as well ;-)

-- Steve