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Re: [libvirt] [PATCH] nodeinfo: Get the correct CPU number on AMD Magny Cours platform



On Fri, May 11, 2012 at 10:28:21PM +0800, Osier Yang wrote:
> "Instead of developing one CPU with 12 cores, the Magny Cours is
> actually two 6 core “Bulldozer” CPUs combined in to one package"
> 
> I.e, each package has two NUMA nodes, and the two numa nodes share
> the same core ID set (0-6), which means parsing the cores number
> from sysfs doesn't work in this case.
> 
> And the wrong CPU number could cause three problems for libvirt:
> 
> 1) performance lost
> 
>   A domain without "cpuset" or "placement='auto'" (to drive numad)
> specified will be only pinned to part of the CPUs.
> 
> 2) domain can be started
> 
>   If a domain uses numad, and the advisory nodeset returned from
> numad contains node which exceeds the range of wrong total CPU
> number. The domain will fail to start, as the bitmask passed to
> sched_setaffinity could be fully filled with zero.
> 
> 3) wrong CPU number affects lots of stuffs.
> 
>   E.g. for command "virsh vcpuinfo", "virsh vcpupin", it will always
> output with the truncated CPU list.
> 
> For more details:
> 
> https://www.redhat.com/archives/libvir-list/2012-May/msg00607.html
> 
> This patch is to fix the problem by parsing /proc/cpuinfo to get
> the value of field "cpu cores", and use it as nodeinfo->cores if
> it's greater than the cores number from sysfs.
> ---
>  src/nodeinfo.c |   28 ++++++++++++++++++++++++++++
>  1 files changed, 28 insertions(+), 0 deletions(-)

Given how complex (and frequently broken) this CPU info parsing is, I
really think this change needs to be accompanied by improvements to
our test suite. IIUC, we can easily test this by including copies of
the /proc/cpuinfo from a variety of different machines in our tests
directory and running the APIs against that.

Daniel
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