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[Nikola Đipanov <ndipanov@xxxxxxxxxx>]

Public bug reported:

Calculating CPU pinning for an instance for the host with hyperthreading
fails in certain cases. Most notably when the instance has an odd number
of CPUs, due to a bug in the logic we might either fail to pin entirely
or end up avoiding siblings by accident, although the default policy
should be to prefer them (which is what happens when we have an even
number of CPUs).

Consider a host with CPUs [(0, 3), (1, 4), (2, 5)]  (brackets denote
thread siblings). Instance with 5 CPUs would fail to get fitted onto
this host even though it's clear that it's absolutely possible to fit
that instance on there.

Another unexpected result happens when we have a host [[0, 8], [1, 9],
[2, 10], [3, 11], [4, 12], [5, 13], [6, 14], [7, 15]] and a 5 CPU
instance. In this case an instance with 5 CPUs would get pinned as
follows (instance cpu -> host cpu): [(0 -> 0), (1 -> 1), (2 -> 2), (3 ->
3), (4 -> 4)] which is wrong since the default for instances with an
even number of CPUs would be to prefer sibling CPUs.

After inspecting the fitting logic code:

https://github.com/openstack/nova/blob/b0013d93ffeaed53bc28d9558def26bdb7041ed7/nova/virt/hardware.py#L653

I also noticed that we would consult the existing topology of the
instance NUMA cell when deciding on the proper way to fit instance CPUs
onto host. This is actually wrong after
https://review.openstack.org/#/c/198312/. We don't need to consider the
requested topology in the CPU fitting any more as the code that decides
on the final CPU topology takes all of this into account.

** Affects: nova
     Importance: Medium
         Status: New


** Tags: numa

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https://bugs.launchpad.net/bugs/1501358

Title:
  cpu pinning on the host that has siblings does not work properly in
  some cases (instance with odd CPUs)

Status in OpenStack Compute (nova):
  New

Bug description:
  Calculating CPU pinning for an instance for the host with
  hyperthreading fails in certain cases. Most notably when the instance
  has an odd number of CPUs, due to a bug in the logic we might either
  fail to pin entirely or end up avoiding siblings by accident, although
  the default policy should be to prefer them (which is what happens
  when we have an even number of CPUs).

  Consider a host with CPUs [(0, 3), (1, 4), (2, 5)]  (brackets denote
  thread siblings). Instance with 5 CPUs would fail to get fitted onto
  this host even though it's clear that it's absolutely possible to fit
  that instance on there.

  Another unexpected result happens when we have a host [[0, 8], [1, 9],
  [2, 10], [3, 11], [4, 12], [5, 13], [6, 14], [7, 15]] and a 5 CPU
  instance. In this case an instance with 5 CPUs would get pinned as
  follows (instance cpu -> host cpu): [(0 -> 0), (1 -> 1), (2 -> 2), (3
  -> 3), (4 -> 4)] which is wrong since the default for instances with
  an even number of CPUs would be to prefer sibling CPUs.

  After inspecting the fitting logic code:

  https://github.com/openstack/nova/blob/b0013d93ffeaed53bc28d9558def26bdb7041ed7/nova/virt/hardware.py#L653

  I also noticed that we would consult the existing topology of the
  instance NUMA cell when deciding on the proper way to fit instance
  CPUs onto host. This is actually wrong after
  https://review.openstack.org/#/c/198312/. We don't need to consider
  the requested topology in the CPU fitting any more as the code that
  decides on the final CPU topology takes all of this into account.

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