From mboxrd@z Thu Jan  1 00:00:00 1970
From: Dave Hansen <dave@linux.vnet.ibm.com>
Subject: Re: [PATCH 1/2] sched: add virt sched domain for the guest
Date: Wed, 23 May 2012 08:23:06 -0700
Message-ID: <4FBD00DA.5080308@linux.vnet.ibm.com>
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Cc: Liu ping fan <kernelfans@gmail.com>, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org, qemu-devel@nongnu.org,
	Ingo Molnar <mingo@redhat.com>, Avi Kivity <avi@redhat.com>,
	Anthony Liguori <anthony@codemonkey.ws>
To: Peter Zijlstra <peterz@infradead.org>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <1337762914.9698.65.camel@twins>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: kvm.vger.kernel.org

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. ;)