[libvirt] Problem of host CPU topology parsing

[Daniel P. Berrange]

On Fri, May 11, 2012 at 05:42:34PM +0800, Osier Yang wrote:
> On 2012年05月11日 17:01, Jiri Denemark wrote:
> >On Fri, May 11, 2012 at 10:47:06 +0200, Michal Privoznik wrote:
> >>On 11.05.2012 10:40, Osier Yang wrote:
> >>>     /* nodeinfo->sockets is supposed to be a number of sockets per NUMA
> >>>node,
> >>>      * however if NUMA nodes are not composed of whole sockets, we just lie
> >>>      * about the number of NUMA nodes and force apps to check
> >>>capabilities XML
> >>>      * for the actual NUMA topology.
> >>>      */
> >>>     if (nodeinfo->sockets % nodeinfo->nodes == 0)
> >>>         nodeinfo->sockets /= nodeinfo->nodes;
> >>>     else
> >>>         nodeinfo->nodes = 1;
> >>>
> >>>Jirka said this was for a fix, but I don't quite understand it,
> >>>what does the "nodeinfo.nodes" mean actually? Shouldn't it
> >>>be 8 (for the 48 CPUs machine) instead? But then we will be
> >>>wrong again with using VIR_NODEINFO_MAXCPUS.
> >>
> >>Why do you think it will be wrong? My understanding is that
> >>VIR_NODEINFO_MAXCPUS just tell the max number of possible cpus not the
> >>actual. So if it's over 48 we are safe.
> >
> >Not really, the macro should count exactly the number of CPUs available to
> >host, otherwise lots of other issues (incl. backward compatibility) appear. It
> >is just a badly named macro that should never exist but we can't do anything
> >with it since it is our public API.
> >
> >>Btw: the code above seems like a hack to me.
> >
> >Yes, it is a hack but it's unfortunately required because we can't change the
> >macro.
> >
> >Anyway, I agree with Daniel that the bug most likely lies somewhere in the
> >code that populates nodeinfo structure.
> >
> >Jirka
> 
> In /proc/cpuinfo:
> 
> <snip>
> cpu cores       : 12
> </snip>
> 
> However, there are only 6 core IDs, as showed in
> http://fpaste.org/mtoA/. And we parse the core_id
> file of each CPU as:
> 
>         core = parse_core(cpu);
>         if (!CPU_ISSET(core, &core_mask)) {
>             CPU_SET(core, &core_mask);
>             nodeinfo->cores++;
>         }
> 
> and thus get only 6 cores. Don't known how 12 in /proc/cpuinfo
> is figured out. But could it be a clue?

Ahhh.  The AMD 12 "core" CPUs are in fact a pair of 6 core CPUs
with 2 NUMA nodes in the CPU itself.

http://frankdenneman.nl/2011/01/amd-magny-cours-and-esx/

 "Instead of developing one CPU with 12 cores, the Magny Cours is
  actually two 6 core “Bulldozer” CPUs combined in to one package."

So we need to take account of this when calculating the NUMA nodes

Daniel
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