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[Elod Illes <1889257@xxxxxxxxxxxxxxxxxx>]

** Also affects: nova/train
   Importance: Undecided
       Status: New

** Also affects: nova/ussuri
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1889257

Title:
  Live migration of realtime instances is broken

Status in OpenStack Compute (nova):
  Fix Released
Status in OpenStack Compute (nova) train series:
  New
Status in OpenStack Compute (nova) ussuri series:
  New

Bug description:
  Attempting to live migrate an instance with realtime enabled fails on
  master (commit d4c857dfcb1). This appears to be a bug with the live
  migration of pinned instances feature introduced in Train.

  # Steps to reproduce

  Create a server using realtime attributes and then attempt to live
  migrate it. For example:

    $ openstack flavor create --ram 1024 --disk 0 --vcpu 4 \
      --property 'hw:cpu_policy=dedicated' \
      --property 'hw:cpu_realtime=yes' \
      --property 'hw:cpu_realtime_mask=^0-1' \
      realtime

    $ openstack server create --os-compute-api-version=2.latest \
      --flavor realtime --image cirros-0.5.1-x86_64-disk --nic none \
      --boot-from-volume 1 --wait \
      test.realtime

    $ openstack server migrate --live-migration test.realtime

  # Expected result

  Instance should be live migrated.

  # Actual result

  The live migration never happens. Looking at the logs we see the
  following error:

    Traceback (most recent call last):
      File "/usr/local/lib/python3.6/dist-packages/eventlet/hubs/hub.py", line 461, in fire_timers
        timer()
      File "/usr/local/lib/python3.6/dist-packages/eventlet/hubs/timer.py", line 59, in __call__
        cb(*args, **kw)
      File "/usr/local/lib/python3.6/dist-packages/eventlet/event.py", line 175, in _do_send
        waiter.switch(result)
      File "/usr/local/lib/python3.6/dist-packages/eventlet/greenthread.py", line 221, in main
        result = function(*args, **kwargs)
      File "/opt/stack/nova/nova/utils.py", line 670, in context_wrapper
        return func(*args, **kwargs)
      File "/opt/stack/nova/nova/virt/libvirt/driver.py", line 8966, in _live_migration_operation
        #     is still ongoing, or failed
      File "/usr/local/lib/python3.6/dist-packages/oslo_utils/excutils.py", line 220, in __exit__
        self.force_reraise()
      File "/usr/local/lib/python3.6/dist-packages/oslo_utils/excutils.py", line 196, in force_reraise
        six.reraise(self.type_, self.value, self.tb)
      File "/usr/local/lib/python3.6/dist-packages/six.py", line 703, in reraise
        raise value
      File "/opt/stack/nova/nova/virt/libvirt/driver.py", line 8959, in _live_migration_operation
        #  2. src==running, dst==paused
      File "/opt/stack/nova/nova/virt/libvirt/guest.py", line 658, in migrate
        destination, params=params, flags=flags)
      File "/usr/local/lib/python3.6/dist-packages/eventlet/tpool.py", line 190, in doit
        result = proxy_call(self._autowrap, f, *args, **kwargs)
      File "/usr/local/lib/python3.6/dist-packages/eventlet/tpool.py", line 148, in proxy_call
        rv = execute(f, *args, **kwargs)
      File "/usr/local/lib/python3.6/dist-packages/eventlet/tpool.py", line 129, in execute
        six.reraise(c, e, tb)
      File "/usr/local/lib/python3.6/dist-packages/six.py", line 703, in reraise
        raise value
      File "/usr/local/lib/python3.6/dist-packages/eventlet/tpool.py", line 83, in tworker
        rv = meth(*args, **kwargs)
      File "/usr/local/lib/python3.6/dist-packages/libvirt.py", line 1745, in migrateToURI3
        if ret == -1: raise libvirtError ('virDomainMigrateToURI3() failed', dom=self)
    libvirt.libvirtError: vcpussched attributes 'vcpus' must not overlap

  Looking further, we see there are issues with the XML we are
  generating for the destination. Compare what we have on the source
  before updating the XML for the destination:

    DEBUG nova.virt.libvirt.migration [-] _update_numa_xml input xml=<domain type="kvm">
      ...
      <cputune>
        <shares>4096</shares>
        <vcpupin vcpu="0" cpuset="0"/>
        <vcpupin vcpu="1" cpuset="1"/>
        <vcpupin vcpu="2" cpuset="4"/>
        <vcpupin vcpu="3" cpuset="5"/>
        <emulatorpin cpuset="0-1"/>
        <vcpusched vcpus="2" scheduler="fifo" priority="1"/>
        <vcpusched vcpus="3" scheduler="fifo" priority="1"/>
      </cputune
      ...
    </domain>
     {{(pid=12600) _update_numa_xml /opt/stack/nova/nova/virt/libvirt/migration.py:97}

  To what we have after the update:

    DEBUG nova.virt.libvirt.migration [-] _update_numa_xml output xml=<domain type="kvm">
      ...
      <cputune>
        <shares>4096</shares>
        <vcpupin vcpu="0" cpuset="0"/>
        <vcpupin vcpu="1" cpuset="1"/>
        <vcpupin vcpu="2" cpuset="4"/>
        <vcpupin vcpu="3" cpuset="5"/>
        <emulatorpin cpuset="0-1"/>
        <vcpusched vcpus="2-3" scheduler="fifo" priority="1"/>
        <vcpusched vcpus="3" scheduler="fifo" priority="1"/>
      </cputune>
      ...
    </domain>
     {{(pid=12600) _update_numa_xml /opt/stack/nova/nova/virt/libvirt/migration.py:131}}

  The issue is the 'vcpusched' elements. We're assuming there are only
  one of these elements when updating the XML for the destination [1].
  Have to figure out why there are multiple elements and how best to
  handle this (likely by deleting and recreating everything).

  I suspect the reason we didn't spot this is because libvirt is
  rewriting the XML on us. This is what nova is providing libvirt upon
  boot:

    DEBUG nova.virt.libvirt.driver [...] [instance: ...] End _get_guest_xml xml=<domain type="kvm">
      ...
      <cputune>
        <shares>4096</shares>
        <emulatorpin cpuset="0-1"/>
        <vcpupin vcpu="0" cpuset="0"/>
        <vcpupin vcpu="1" cpuset="1"/>
        <vcpupin vcpu="2" cpuset="4"/>
        <vcpupin vcpu="3" cpuset="5"/>
        <vcpusched vcpus="2-3" scheduler="fifo" priority="1"/>
      </cputune>
      ...
    </domain>
     {{(pid=12600) _get_guest_xml /opt/stack/nova/nova/virt/libvirt/driver.py:6331}}

  but that's changed by time we get to recalculating things.

  The solution is probably to remove all 'vcpusched' elements and
  recreate them, rather than trying to update stuff inline.

  [1]
  https://github.com/openstack/nova/blob/21.0.0/nova/virt/libvirt/migration.py#L152-L155

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