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

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Message-ID: <4FBD00DA.5080308@linux.vnet.ibm.com>
Date:	Wed, 23 May 2012 08:23:06 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Liu ping fan <kernelfans@...il.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, qemu-devel@...gnu.org,
	Ingo Molnar <mingo@...hat.com>, Avi Kivity <avi@...hat.com>,
	Anthony Liguori <anthony@...emonkey.ws>
Subject: Re: [PATCH 1/2] sched: add virt sched domain for the guest

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. ;)

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