'hw:cpu_thread_policy=prefer' misbehaviour

Description
===========

'hw:cpu_thread_policy=prefer' allocates vCPUs in pairs of sibling threads properly. An odd number of vCPUs will allocate pairs and a single one. That single one should not be isolated. So 20 available threads, shall be able to allocate 4 VMs of 5 vCPUs. When booting up the third VM, it is giving an error.

Steps to reproduce
==================

1.- Creating a flavor:

nova flavor-create pinning auto 1024 10 5
nova flavor-key pinning set hw:cpu_policy=dedicated
nova flavor-key pinning set hw:cpu_thread_policy=prefer
nova flavor-key pinning set hw:numa_nodes=1

2.- Booting up simple VMs:

nova boot testPin1 --flavor pinning --image cirros --nic net-id=$NET_ID

In my setup, I have 20 available threads:

  NUMANode L#0 (P#0 32GB)
    Socket L#0 + L3 L#0 (15MB)
      L2 L#0 (256KB) + L1d L#0 (32KB) + L1i L#0 (32KB) + Core L#0
        PU L#0 (P#0)
        PU L#1 (P#12)
      L2 L#1 (256KB) + L1d L#1 (32KB) + L1i L#1 (32KB) + Core L#1
        PU L#2 (P#2)
        PU L#3 (P#14)
      L2 L#2 (256KB) + L1d L#2 (32KB) + L1i L#2 (32KB) + Core L#2
        PU L#4 (P#4)
        PU L#5 (P#16)
      L2 L#3 (256KB) + L1d L#3 (32KB) + L1i L#3 (32KB) + Core L#3
        PU L#6 (P#6)
        PU L#7 (P#18)
      L2 L#4 (256KB) + L1d L#4 (32KB) + L1i L#4 (32KB) + Core L#4
        PU L#8 (P#8)
        PU L#9 (P#20)
      L2 L#5 (256KB) + L1d L#5 (32KB) + L1i L#5 (32KB) + Core L#5
        PU L#10 (P#10)
        PU L#11 (P#22)

  NUMANode L#1 (P#1 32GB) + Socket L#1 + L3 L#1 (15MB)
    L2 L#6 (256KB) + L1d L#6 (32KB) + L1i L#6 (32KB) + Core L#6
      PU L#12 (P#1)
      PU L#13 (P#13)
    L2 L#7 (256KB) + L1d L#7 (32KB) + L1i L#7 (32KB) + Core L#7
      PU L#14 (P#3)
      PU L#15 (P#15)
    L2 L#8 (256KB) + L1d L#8 (32KB) + L1i L#8 (32KB) + Core L#8
      PU L#16 (P#5)
      PU L#17 (P#17)
    L2 L#9 (256KB) + L1d L#9 (32KB) + L1i L#9 (32KB) + Core L#9
      PU L#18 (P#7)
      PU L#19 (P#19)
    L2 L#10 (256KB) + L1d L#10 (32KB) + L1i L#10 (32KB) + Core L#10
      PU L#20 (P#9)
      PU L#21 (P#21)
    L2 L#11 (256KB) + L1d L#11 (32KB) + L1i L#11 (32KB) + Core L#11
      PU L#22 (P#11)
      PU L#23 (P#23)

Using the cpu_thread_policy:prefer, the behaviour is ok for the first two VMs. So 5 threads are allocated in pairs.

[root@nfvsdn-04 ~(keystone_admin)]# virsh vcpupin 2
VCPU: CPU Affinity
----------------------------------
   0: 10
   1: 22
   2: 16
   3: 4
   4: 8

[root@nfvsdn-04 ~(keystone_admin)]# virsh vcpupin 3
VCPU: CPU Affinity
----------------------------------
   0: 17
   1: 5
   2: 3
   3: 15
   4: 11

However, eventhough there are enough threads in order to allocate another 2 VMs with the same flavor, I get the following error booting up the third VM:

INFO nova.filters Filtering removed all hosts for the request with instance ID 'cbb53e29-a7da-4c14-a3ad-4fb3aa04f101'. Filter results: ['RetryFilter: (start: 1, end: 1)', 'AvailabilityZoneFilter: (start: 1, end: 1)', 'RamFilter: (start: 1, end: 1)', 'C[r│omputeFilter: (start: 1, end: 1)', 'ComputeCapabilitiesFilter: (start: 1, end: 1)', 'ImagePropertiesFilter: (start: 1, end: 1)', 'CoreFilter: (start: 1, end: 1)', 'NUMATopologyFilter: (start: 1, endo: 0)']

There should be enough space for 4 VMs allocated with cpu_thread_policy=prefer flavor.

Expected result
===============

To have 4 VMs up&running with flavor 'pinning'.

Actual result
=============

3rd VM fails at scheduling.

Environment
==========

All-in-one environment.