Topology updates and NUMA-level sched domains

[Peter Zijlstra]

On Wed, Apr 08, 2015 at 12:32:01PM +0200, Brice Goglin wrote:
> Le 07/04/2015 21:41, Peter Zijlstra a écrit :
> > No, that's very much not the same. Even if it were dealing with hotplug
> > it would still assume the cpu to return to the same node.
> >
> > But mostly people do not even bother to handle hotplug.
> >
> 
> You said userspace assumes the cpu<->node relation is a boot-time fixed
> one, and hotplug breaks this.

I said no such thing. Regular hotplug actually respects that relation.

> How do you expect userspace to handle hotplug?

Mostly not. Why would they? CPU hotplug is rare and mostly a case of:
don't do that then.

Its just that some of the virt wankers are using it for resource
management which is entirely misguided. Then again, most of virt is.

> Is there a convenient way to be notified when a CPU (or memory)
> is unplugged?

I think you can poll some sysfs file or other.