Re: [Qemu-devel] [PATCH 1/2] sched: add virt sched domain for the guest

[Peter Zijlstra <peterz@infradead.org>]

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.