Re: [PATCH -tip/core/rcu 3/6] Simplify RCU CPU-hotplug notification

Paul E. McKenney
2009-08-20T04:21:27Z

On Thu, Aug 20, 2009 at 12:02:44PM +0800, Lai Jiangshan wrote:
> Paul E. McKenney wrote:
> > From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> >
> > Use the new cpu_notifier() API to simplify RCU's CPU-hotplug
> > notifiers, collapsing down to a single such notifier. This
> > makes it trivial to provide the notifier-ordering guarantee
> > that rcu_barrier() depends on. Also remove redundant
> > open_softirq() calls from Hierarchical RCU notifier.
> >
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > ---
> > kernel/rcupdate.c | 16 +++++++++++++++-
> > kernel/rcupreempt.c | 25 ++-----------------------
> > kernel/rcutree.c | 17 +++++------------
> > 3 files changed, 22 insertions(+), 36 deletions(-)
> >
> > diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
> > index eae29c2..8df1156 100644
> > --- a/kernel/rcupdate.c
> > +++ b/kernel/rcupdate.c
> > @@ -217,9 +217,13 @@ static void rcu_migrate_callback(struct rcu_head *notused)
> > wake_up(&rcu_migrate_wq);
> > }
> >
> > +extern int rcu_cpu_notify(struct notifier_block *self,
> > + unsigned long action, void *hcpu);
> > +
> > static int __cpuinit rcu_barrier_cpu_hotplug(struct notifier_block *self,
> > unsigned long action, void *hcpu)
> > {
> > + rcu_cpu_notify(self, action, hcpu);
> > if (action == CPU_DYING) {
> > /*
> > * preempt_disable() in on_each_cpu() prevents stop_machine(),
> > @@ -244,8 +248,18 @@ static int __cpuinit rcu_barrier_cpu_hotplug(struct notifier_block *self,
> >
> > void __init rcu_init(void)
> > {
> > + int i;
> > +
> > __rcu_init();
> > - hotcpu_notifier(rcu_barrier_cpu_hotplug, 0);
> > + cpu_notifier(rcu_barrier_cpu_hotplug, 0);
> > +
> > + /*
> > + * We don't need protection against CPU-hotplug here because
> > + * this is called early in boot, before either interrupts
> > + * or the scheduler are operational.
> > + */
> > + for_each_online_cpu(i)
> > + rcu_barrier_cpu_hotplug(NULL, CPU_UP_PREPARE, (void *)(long)i);
>
> I think,
> There is only one cpu in cpu online map at early boot time,
> it is the current cpu.

Indeed there is. But I wouldn't put it past the fast-boot guys to
parallelize early boot. ;-)

Thanx, Paul