Re: [PATCH 01/51] CPU hotplug: Provide lockless versions of callback registration functions

[Toshi Kani <toshi.kani@hp.com>]

On Thu, 2014-02-13 at 10:56 +0000, Srivatsa S. Bhat wrote:
> On 02/12/2014 11:48 AM, Srivatsa S. Bhat wrote:
: 
> >>> For example, something like the code snippet shown below looks pretty
> >>> neat to me:
> >>>
> >>> 	cpu_notifier_register_begin();
> >>>
> >>> 	for_each_online_cpu(cpu)
> >>> 		init_cpu(cpu);
> >>>
> >>> 	register_cpu_notifier(&foobar_cpu_notifier);
> >>>
> >>> 	cpu_notifier_register_done();
> >>>
> >>> What do you think?
> >>
> >> I agree that it is cleaner for the callers as long as people understand
> >> how to use them.  Can you document them properly so that they know when
> >> they need to use them instead of the familiar get/put_online_cpus()?
> >>
> > 
> > Sure.. I had updated the documentation with the semantics introduced in
> > this patchset, in patch 2:
> > 
> > http://thread.gmane.org/gmane.linux.kernel/1641638/focus=1641695
> > 
> > Similarly I'll keep the docs updated with these new APIs in v2 as well.
> >
> 
> For now, however, let us not add the new rw-semaphore to the CPU hotplug
> core yet. Its very unlikely that we'll see any performance issue immediately,
> due to serialized initialization of cpu hotplug notifiers, since early boot
> is mostly sequential anyway.
> 
> Some time in the future, if we start hitting bottlenecks in the cpu hotplug
> notifier registration phase (perhaps when we implement parallel CPU boot-up
> infrastructure), then we can directly use the rw-semaphore solution, since
> we have already worked it out. Besides, like Gautham said, we might want
> to be more careful and have a very good justification before adding more
> locks to the CPU hotplug core code. So we'll add the new rw-sempahore if
> and when it becomes necessary.
> 
> I'll post the v2 with the earlier design itself, by adding the new symbols
> cpu_notifier_register_begin/done() (to enhance the readability) and map
> them to cpu_maps_update_begin/done().

Sounds reasonable to me.  I was also concerned about exporting and
overloading cpu_maps_update_begin/done() for a different purpose (their
purpose is to update cpu_maps).  So, I think adding the new interfaces
is good when we cannot use get/set_online_cpus() for this.

Thanks,
-Toshi