On Wed, 2012-05-23 at 08:23 -0700, Dave Hansen wrote: > On 05/23/2012 01:48 AM, Peter Zijlstra wrote: > > On Wed, 2012-05-23 at 16:34 +0800, Liu ping fan wrote: > >> > so we need to migrate some of vcpus from node-B to node-A, or to > >> > node-C. > > This is absolutely broken, you cannot do that. > > > > A guest task might want to be node affine, it looks at the topology sets > > a cpu affinity mask and expects to stay on that node. > > > > But then you come along, and flip one of those cpus to another node. The > > guest task will now run on another node and get remote memory accesses. > > Insane, sure. But, if the node has physically gone away, what do we do? > I think we've got to either kill the guest, or let it run somewhere > suboptimal. Sounds like you're advocating killing it. ;) You all seem terribly confused. If you want a guest that 100% mirrors the host topology you need hard-binding of all vcpu threads and clearly you're in trouble if you unplug a host cpu while there's still a vcpu expecting to run there. That's an administrator error and you get to keep the pieces, I don't care. In case you want simple virt-numa where a number of vcpus constitute a vnode and have their memory all on the same node the vcpus are ran on, what does it matter if you unplug something in the host? Just migrate everything -- including memory. But what Liu was proposing is completely insane and broken. You cannot simply remap cpu:node relations. Wanting to do that shows a profound lack of understanding. Our kernel assumes that a cpu remains on the same node. All userspace that does anything with NUMA assumes the same. You cannot change this. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
