[libvirt] [RFC PATCH v2 1/4] numa: describe siblings distances within cells

[Wim ten Have]

On Wed, 28 Jun 2017 15:21:29 +0100
"Daniel P. Berrange" <berrange at redhat.com> wrote:

> On Wed, Jun 28, 2017 at 03:56:36PM +0200, Wim Ten Have wrote:
> > From: Wim ten Have <wim.ten.have at oracle.com>
> > 
> > Add libvirtd NUMA cell domain administration functionality to
> > describe underlying cell id sibling distances in full fashion
> > when configuring HVM guests.
> > 
> > [below is an example of a 4 node setup]

  Hi Daniel, I think you're best to consult on below issue
  too. This given it is related to the changes I am preparing
  but was obviously not yet discussed. Your other hint towards
  symmetry on distance configuration and autonomous actions
  made me more carefully approach whole.

  Working <numa> <distance> support, specifically for xenlight
  driver I for certain topic related detail consult QEMU/KVM
  behavior.  Here when describing a guest XML configuration
  in libvirt there's this nuance when <numa> comes into the
  game. Imagine below.

      <vcpu placement='static' current='5'>8</vcpu>
      	...
      <cpu mode='host-passthrough'>
        <numa>
          <cell id='0' cpus='0,2' memory='262144' unit='KiB'/>
          <cell id='1' cpus='1,3' memory='262144' unit='KiB'/>
        </numa>
      </cpu>

  Running QEMU/KVM with such onlines 5 CPUs.  This is wrong IMHO.

      <wtenhave at kvm26:35> lscpu
      Architecture:        x86_64
      CPU op-mode(s):      32-bit, 64-bit
      Byte Order:          Little Endian
      CPU(s):              5
      On-line CPU(s) list: 0-4
	...
      NUMA node(s):        2
	...
      NUMA node0 CPU(s):   0,2,4
      NUMA node1 CPU(s):   1,3

  My thinking is that <vcpu ... current='_5_' had to be
  computed somewhere around under;
	 virDomainDefParseXML() -> virDomainDefPostParse()
  path and forcibly put to '4'.

      <vcpu placement='static' current='4'>8</vcpu>

  This is of course a <numa> related issue.

  Is it worth fixing this (QEMU/KVM) case?  If doing that would
  that potentially hurt specific use and that way libvirt?

- Wim.


-- 
Wim ten Have | Consulting Member of Technical Staff
Oracle Linux and VM Development Engineering
ORACLE Nederland BV | Hertogswetering 163-167 | 3543 AS Utrecht/NL