Comment 3 for bug 1578155

Reviewed: https://review.openstack.org/342709
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=8361d8d6c315bb3ae71c3ff0147f7d5156bc46f3
Submitter: Jenkins
Branch: master

commit 8361d8d6c315bb3ae71c3ff0147f7d5156bc46f3
Author: Stephen Finucane <email address hidden>
Date: Tue Jul 19 14:01:53 2016 -0700

    Allow linear packing of cores

    Given the following single-socket, four-core, HT-enabled CPU topology:

       +---+---+ +---+---+ +---+---+ +---+---+
       | x | x | | x | | | x | | | | |
       +---+---+ +---+---+ +---+---+ +---+---+
         1 4 2 5 3 6 4 7

    Attempting to boot an instance with four cores and no explicit
    'cpu_thread_policy' should be successful, with cores 5,6,4,7 used.
    However, the current implementation of this implicit policy attempts to
    fit the same number of instance cores onto each host CPU. For example,
    a four core instance would result in either a 2*2 layout (two instance
    cores on each of two host CPUs), or a 1*4 layout (one instance core on
    each of four host CPUs). This may be correct behavior *where possible*,
    but if this is not possible then any and all cores should be used.

    Resolve this issue by adding a fallthrough case, whereby if the
    standard fitting policy fails, a linear assignment is used to properly
    fit the instance cores.

    Change-Id: I73f7f771b7514060f1f74066e3dea1da8fe74c21
    Closes-Bug: #1578155
    mitaka-backport-potential