Re: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE_RCU from softirq to kthread

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2011-02-28 at 11:29 +0800, Lai Jiangshan wrote:
> >>> +static int rcu_cpu_kthread_should_stop(int cpu)
> >>> +{
> >>> +   while (cpu_is_offline(cpu) || smp_processor_id() != cpu) {
> >>> +           if (kthread_should_stop())
> >>> +                   return 1;
> >>> +           local_bh_enable();
> >>> +           schedule_timeout_uninterruptible(1);
> >>> +           if (smp_processor_id() != cpu)
> >>> +                   set_cpus_allowed_ptr(current, cpumask_of(cpu));
> >>
> >> The current task is PF_THREAD_BOUND,
> >> Why do "set_cpus_allowed_ptr(current, cpumask_of(cpu));" ?
> > 
> > Because I have seen CPU hotplug operations unbind PF_THREAD_BOUND threads.

Correct, but that's on unplug, the rest of the story seems about plug,
so just detatch the thread on down/offline and let it die when its done.

> > In addition, I end up having to spawn the kthread at CPU_UP_PREPARE time,

Sure, that's a common time to create such treads :-), you can
kthread_ceate()+kthread_bind() in UP_PREPARE, just don't wake them yet.

> > at which point the thread must run unbound because its CPU isn't online
> > yet.  I cannot invoke kthread_create() within the stop-machine handler
> > (right?).

No you can not ;-)

>   I cannot wait until CPU_ONLINE time because that results in
> > hangs when other CPU notifiers wait for grace periods.
> > 
> > Yes, I did find out about the hangs the hard way.  Why do you ask?  ;-)

Right, so I assume that whoever needs the thread will:

 1) wake the thread,
 2) only do so after the cpu is actually online, how else could it be
executing code? :-)