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[Thierry Carrez <thierry.carrez+lp@xxxxxxxxx>]

** Changed in: nova
       Status: Fix Committed => Fix Released

** Changed in: nova
    Milestone: None => liberty-2

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https://bugs.launchpad.net/bugs/1471187

Title:
  CPU topology does not honour cpu_max_threads when NUMA topology is
  used

Status in OpenStack Compute (nova):
  Fix Released

Bug description:
  Consider a flavour with 4 vcpus, and image property
  hw:cpu_threads==2. Now the NUMA topology may prefer a thread count of
  4, but best of the thread preference set, the guest will end up with 2
  sockets, 1 core and 2 threads.

  Now consider a flavour with 4 vcpus, and image property
  hw:cpu_max_threads==2. Now the NUMA topology may prefer a thread count
  of 4, but best of the thread limit is set, so the guest should again
  end up with 2 sockets, 1 core and 2 threads.

  The current code in nova.virt.hardware._get_possible_cpu_topologies()
  is not honouring the cpu_max_threads setting in this scenario though,
  so the guest ends up with 1 socket, 1 core and 4 threads. This
  violates the cpu_max_threads setting.

  
  Meanwhile if the vCPU count is 6 and the NUMA topology prefers a thread count of 4, then the code is actually incapable of coming up with a valid topology because it only considers topologies with an exact thread count of 4 and 4 does not divide into 6. 

  nova.exception.ImageVCPULimitsRangeImpossible: Requested vCPU limits
  6:6:6 are impossible to satisfy for vcpus count 6

  which is clearly a bogus error as you can easily satisfy that with
  6:1:1, 1:6:1, 1:1:6, 2:1:3, 2:3:1, 3:2:1, 3:1:2, 1:2:3, 1:3:2, so it
  should have picked 2:1:3, as that has thread count closest matching
  the NUMA topology and the maximum socket count.

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