[libvirt] [PATCH v1 1/1] virhostcpu.c: consider empty NUMA nodes in topology validation

[Andrea Bolognani]

On Fri, 2019-02-08 at 14:14 +0100, Jiri Denemark wrote:
> On Fri, Feb 08, 2019 at 11:03:34 -0200, Daniel Henrique Barboza wrote:
> > Some devices creates empty (= cpu-less) NUMA nodes to host
> > its memory. This results in topologies where the following
> > sanity rule does not apply as is:
> > 
> > nodes * sockets * cores * threads = total_cpus
> > 
> > As a result, a call to 'virsh nodeinfo' will return the default
> > value (1) to nodes, sockets and threads, while cores defaults
> > to the total_cpus value. For example, in a Power9 host that has
> > 160 total cpus, 4 cpu-less NUMA nodes, 2 populated NUMA nodes,
> > 1 socket per populated node, 20 cores per socket and 4 threads
> > per socket:
> > 
> > $ virsh nodeinfo
> > CPU model:           ppc64le
> > CPU(s):              160
> > CPU frequency:       3783 MHz
> > CPU socket(s):       1
> > Core(s) per socket:  160
> > Thread(s) per core:  1
> > NUMA cell(s):        1
> > Memory size:         535981376 KiB
> > 
> > This patch adjusts virHostCPUGetInfoPopulateLinux to count the
> > cpu-less NUMA nodes and discard them in the sanity rule, changing
> > it to:
> > 
> > (nodes - empty_nodes) * sockets * cores * threads = total_cpus
> > 
> > And with this new rule, virsh nodeinfo will return the
> > appropriate info for those topologies, without changing the
> > behavior for any other scenario it was previously working.
> > 
> > This is the resulting output of nodeinfo after this patch in the
> > same Power9 host mentioned above:
> > 
> > $ virsh nodeinfo
> > CPU model:           ppc64le
> > CPU(s):              160
> > CPU frequency:       3783 MHz
> > CPU socket(s):       1
> > Core(s) per socket:  20
> > Thread(s) per core:  4
> > NUMA cell(s):        6
> > Memory size:         535981376 KiB
> 
> This change would break backward compatibility as we have the following
> public macro in libvirt-host.h:
> 
> # define VIR_NODEINFO_MAXCPUS(nodeinfo) \
> ((nodeinfo).nodes*(nodeinfo).sockets*(nodeinfo).cores*(nodeinfo).threads)
> 
> Anyway, the virNodeInfo structure is just not flexible enough to deal
> with all possible topologies and users are advised to look at the host
> capabilities XML to get a proper view of the host CPU topology.

Correct, and it's even documented as such:

  https://libvirt.org/html/libvirt-libvirt-host.html#virNodeInfo

So, as long as the number of CPUs is reported correctly (which it
seems to be) and 'virsh capabilities' doesn't report incorrect
information in the <topology> element, then there's nothing to be
fixed - except for applications that still use virNodeInfo ;)

-- 
Andrea Bolognani / Red Hat / Virtualization