Re: [PATCH -tip/core/rcu 2/6] Introduce cpu_notifier() to handle !HOTPLUG_CPU case

Paul E. McKenney
2009-08-17T18:29:05Z

On Mon, Aug 17, 2009 at 10:21:59AM -0700, Josh Triplett wrote:
> On Sat, 2009-08-15 at 09:53 -0700, Paul E. McKenney wrote:
> > From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> >
> > This patch introduces a new cpu_notifier() API that is similar to
> > hotcpu_notifier(), but which also notifies of CPUs coming online during
> > boot in the !HOTPLUG_CPU case.
> [...]
> > --- a/include/linux/cpu.h
> > +++ b/include/linux/cpu.h
> > @@ -48,6 +48,15 @@ struct notifier_block;
> >
> > #ifdef CONFIG_SMP
> > /* Need to know about CPUs going up/down? */
> > +#if defined(CONFIG_HOTPLUG_CPU) || !defined(MODULE)
> > +#define cpu_notifier(fn, pri) { \
> > + static struct notifier_block fn##_nb __cpuinitdata = \
> > + { .notifier_call = fn, .priority = pri }; \
> > + register_cpu_notifier(&fn##_nb); \
> > +}
> > +#else /* #if defined(CONFIG_HOTPLUG_CPU) || !defined(MODULE) */
> > +#define cpu_notifier(fn, pri) do { (void)(fn); } while (0)
> > +#endif /* #else #if defined(CONFIG_HOTPLUG_CPU) || !defined(MODULE) */
>
> These two definitions seem inconsistent. I think the first one needs to
> change to use do { ... } while(0) as well, so it eats the subsequent
> semicolon.

Ha! I just copied the old hotcpu_notifier style. Will send a patch
fixing both. Or feel free to send one, if you wish.

> Does this really want to live under defined(CONFIG_HOTPLUG_CPU)? What
> happens when onlining CPUs during the !define(CONFIG_HOTPLUG_CPU) case?
> This seems somewhat inconsistent with the explanation in your commit
> message; can you clarify?

The !MODULE covers the !define(CONFIG_HOTPLUG_CPU) case, please see
below.

> Also, why !defined(MODULE)?

Here is how the cases lay out:

o !CONFIG_SMP: there is only one CPU, and so there can be no
CPU-hotplug operations, even at boot. Therefore, the
cpu_notifier() need do nothing.

o CONFIG_SMP && CPU_HOTPLUG_CPU: CPUs can come and go at any
time, so we need cpu_notifier() to actually register a
notifier.

o CONFIG_SMP && !CPU_HOTPLUG_CPU && MODULE: CPUs cannot go
offline, but they do come online at boot time. But MODULE means
that this code is in a module, and modules cannot be loaded
until later, after all CPUs have come online. So cpu_notifier()
need do nothing in this case.

o CONFIG_SMP && !CPU_HOTPLUG_CPU && !MODULE: CPUs cannot go
offline, but they do come online at boot time. This code
is not in a module, so might be running at boot time, and
thus might need to deal with CPUs coming online. Therefore,
cpu_notifier() must actually register a notifier.

The difference between hotcpu_notifier() and cpu_notifier() is in this
last case. You would use hotcpu_notifier() for non-module code that did
not run until all CPUs had come online, and thus would not need to deal
with CPU hotplug unless HOTPLUG_CPU was actually defined.

Thanx, Paul