On Wed, Nov 13, 2013 at 11:23:38AM -0500, Steven Rostedt wrote:
> On Wed, 13 Nov 2013 08:18:29 -0800
> "Paul E. McKenney" <[email protected]> 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 <[email protected]> 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.
Based on Thomas's most recent response, it sounds like I need to check
a frozen shark or something. ;-)
Thanx, Paul
> That is, tick_nohz_enabled is set at bootup and never changes.
>
> Perhaps this old hardware uncovered other bugs as well ;-)
>
> -- Steve
>
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