Re: [PATCH 0/6] x86/cpu hotplug: Wake up offline CPU via mwait or nmi

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Jun 05, 2012 at 11:37:21PM +0200, Peter Zijlstra wrote:
> On Tue, 2012-06-05 at 14:29 -0700, Paul E. McKenney wrote:
> > OK, I'll bite...  Why not just use CPU hotplug to expel the timers?
> 
> Currently? Can you say: 'kstopmachine'?

So if CPU hotplug (or whatever you want to call it) stops using
kstopmachine, you are OK with it?

> But its also a question of interface and naming. Do you want to have to
> iterate all cpus in your isolated set, do you want to bring them down
> far enough to physically unplug. Ideally no to both.

For many use cases, it is indeed not necessary to get to a point where
the CPUs could be physically removed from the system.  But CPU-failure
use cases would need the CPU to be fully deactivated.  And many of the
hardware guys tell me that the CPU-failure case will be getting more
common, though I sure hope that they are wrong.

> If you don't bring them down far enough to unplug, should you still be
> calling it hotplug?

I am not too worried about what it is called.  Though "banish to monastery"
would probably be going too far in the other direction.

> Ideally I think there'd be a file in your cpuset which if opened and
> written to will flush all pending bits (timers, workqueues, the lot) and
> return when this is done (and maybe provide O_ASYNC writes to not wait
> for completion).

The mobile guys probably are not too worried about bulk operations yet
because they don't have that many CPUs, but it might be useful elsewhere.

							Thanx, Paul