nova libvirt driver does not handle resizing with 'hw:cpu_policy' flag

[Stephen Finucane]

We conduced a series of resize checks on instances with/without the CPU pinning parameters provided as part of this blueprint:

    http://specs.openstack.org/openstack/nova-specs/specs/juno/approved/virt-driver-cpu-pinning.html

This is related to 1417667 (https://bugs.launchpad.net/nova/+bug/1417667).

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# Testing Configuration

Testing was conducted on a single-node, Fedora 21-based (3.17.8-300.fc21.x86_64) OpenStack instance (built with devstack). Three flavors were created:

* no-pinning ('hw:cpu_policy=shared')
* pinning ('hw:cpu_policy=dedicated' and 'hw:numa-nodes=1')
* pinning-no-numa ('hw:cpu_policy=dedicated')

# Results

## no pinning -> pinning

Doesn't work. Attempting to do this results in little difference in the output XML.

Before:

    <vcpu placement='static' cpuset='10-19,30-39'>10</vcpu>

After:

    <vcpu placement='static' cpuset='0-9,20-29'>10</vcpu>

STEP: Create a VM with a 'no-pinning' flavor. Dump XML with virsh. Resize and confirm to 'pinning' flavor. Dump XML with virsh.

## no pinning -> pinning (without available CPUs)

Doesn't work (due to above). Pinning does not occur so there is no reason for failure.

STEPS: Create three 10-vcpu VMs with a 'pinning' flavor. Create two 10-vcpu VMs with a 'no-pinning' flavor. Resize and confirm one of latter to 'pinning' flavor. Ensure failure.

## pinning w/ no numa topology -> pinning with numa topology

Doesn't work. There is only one (unrelated) change in the output XML:

Before:

    <disk type='file' device='disk'>
        ...
        <backingStore type='file' index='1'>
            <format type='raw'/>
            <source file='/opt/stack/data/nova/instances/_base/598e2b66ae396bb852a504ea4a7471df6b847f8b'/>
            <backingStore/>
        </backingStore>

After:

    <disk type='file' device='disk'>
        ...
        <backingStore/>

STEPS: Create a VM with a 'pinning-no-numa' flavor. Dump XML with virsh. Resize and confirm to 'pinning' flavor. Dump XML with virsh.

## pinning -> no pinning

Doesn't work. Attempting to do this results in no serious change in the output XML. Here's the important elements of the XML for a resized (pinning -> no pinning) instance:

    <vcpu placement='static'>10</vcpu>
    <cputune>
        <shares>10240</shares>
        <vcpupin vcpu='0' cpuset='17'/>
        <vcpupin vcpu='1' cpuset='37'/>
        <vcpupin vcpu='2' cpuset='18'/>
        <vcpupin vcpu='3' cpuset='38'/>
        <vcpupin vcpu='4' cpuset='19'/>
        <vcpupin vcpu='5' cpuset='39'/>
        <vcpupin vcpu='6' cpuset='33'/>
        <vcpupin vcpu='7' cpuset='13'/>
        <vcpupin vcpu='8' cpuset='34'/>
        <vcpupin vcpu='9' cpuset='14'/>
        <emulatorpin cpuset='13-14,17-19,33-34,37-39'/>
    </cputune>
    <numatune>
        <memory mode='strict' nodeset='1'/>
        <memnode cellid='0' mode='strict' nodeset='1'/>
    </numatune>
    <cpu>
        <topology sockets='5' cores='1' threads='2'/>
        <numa>
            <cell id='0' cpus='0-9' memory='4194304' unit='KiB'/>
        </numa>
    </cpu>

Versus the unresized, no pinning instance:

  <vcpu placement='static' cpuset='10-19,30-39'>10</vcpu>
  <cputune>
    <shares>10240</shares>
  </cputune>
  <cpu>
    <topology sockets='10' cores='1' threads='1'/>
  </cpu>