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Under normal circumstances a bit of resource tracking error is generally okay. 
However in the case of CPU pinning it's a major problem because it's not caught 
at instance boot time, so you end up with two instances that both think they 
have exclusive access to one or more host CPUs.

If we get into this scenario it ends up raising a CPUPinningInvalid exception 
during the resource audit, which causes the audit to be aborted.

Chris

On 06/10/2016 02:36 AM, Matthew Booth wrote:
> Yes, this is a race.
>
> However, it's my understanding that this is 'ok'. The resource tracker doesn't
> claim to be 100% accurate at all times, right? Otherwise why would it update
> itself in a period task in the first place. It's my understanding that the
> resource tracker is basically a best effort cache, and that scheduling decisions
> can still fail at the host. The resource tracker will fix itself next time it
> runs via its periodic task.
>
> Matt (not a scheduler person)
>
> On Thu, Jun 9, 2016 at 10:41 PM, Chris Friesen <chris.friesen at windriver.com
> <mailto:chris.friesen at windriver.com>> wrote:
>
>     Hi,
>
>     I'm wondering if we might have a race between live migration and the
>     resource audit.  I've included a few people on the receiver list that have
>     worked directly with this code in the past.
>
>     In _update_available_resource() we have code that looks like this:
>
>     instances = objects.InstanceList.get_by_host_and_node()
>     self._update_usage_from_instances()
>     migrations = objects.MigrationList.get_in_progress_by_host_and_node()
>     self._update_usage_from_migrations()
>
>
>     In post_live_migration_at_destination() we do this (updating the host and
>     node as well as the task state):
>                  instance.host = self.host
>                  instance.task_state = None
>                  instance.node = node_name
>                  instance.save(expected_task_state=task_states.MIGRATING)
>
>
>     And in _post_live_migration() we update the migration status to "completed":
>              if migrate_data and migrate_data.get('migration'):
>                  migrate_data['migration'].status = 'completed'
>                  migrate_data['migration'].save()
>
>
>     Both of the latter routines are not serialized by the
>     COMPUTE_RESOURCE_SEMAPHORE, so they can race relative to the code in
>     _update_available_resource().
>
>
>     I'm wondering if we can have a situation like this:
>
>     1) migration in progress
>     2) We start running _update_available_resource() on destination, and we call
>     instances = objects.InstanceList.get_by_host_and_node().  This will not
>     return the migration, because it is not yet on the destination host.
>     3) The migration completes and we call post_live_migration_at_destination(),
>     which sets the host/node/task_state on the instance.
>     4) In _update_available_resource() on destination, we call migrations =
>     objects.MigrationList.get_in_progress_by_host_and_node().  This will return
>     the migration for the instance in question, but when we run
>     self._update_usage_from_migrations() the uuid will not be in "instances" and
>     so we will use the instance from the newly-queried migration.  We will then
>     ignore the instance because it is not in a "migrating" state.
>
>     Am I imagining things, or is there a race here?  If so, the negative effects
>     would be that the resources of the migrating instance would be "lost",
>     allowing a newly-scheduled instance to claim the same resources (PCI
>     devices, pinned CPUs, etc.)
>
>     Chris
>
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>
>
> --
> Matthew Booth
> Red Hat Engineering, Virtualisation Team
>
> Phone: +442070094448 (UK)
>
>
>
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