Unnecessary instance lazy-loads during rebuild

[Matt Riedemann]

The rebuild code in the compute manager also handles evacuate. Rebuild is rebuild on the same host, no migration. Evacuate is rebuild the instance on another host, and has a migration context.

This code:

https://github.com/openstack/nova/blob/e01ae75d52900d96355dfcb39ef9b136f0c0d5c4/nova/compute/manager.py#L2718

Is using the mutated_migration_context() context manager which lazy-loads numa_topology, pci_requests and pci_devices and then, since self.migration_context isn't set on the instance, yields as a noop.

Seen here:

http://logs.openstack.org/82/471082/1/gate/gate-novaclient-dsvm-functional-neutron-ubuntu-xenial/796acb7/logs/screen-n-cpu.txt.gz#_Jun_06_13_14_02_547424

Jun 06 13:14:02.547424 ubuntu-xenial-infracloud-chocolate-9158824 nova-compute[20994]: DEBUG nova.objects.instance [None req-5b3770c1-d332-4875-8933-97de8a9890b4 admin admin] Lazy-loading 'pci_requests' on Instance uuid 573258a4-9416-4e13-a765-7c90683f3526 {{(pid=20994) obj_load_attr /opt/stack/new/nova/nova/objects/instance.py:1038}}
Jun 06 13:14:02.562243 ubuntu-xenial-infracloud-chocolate-9158824 nova-compute[20994]: DEBUG nova.objects.instance [None req-5b3770c1-d332-4875-8933-97de8a9890b4 admin admin] Lazy-loading 'pci_devices' on Instance uuid 573258a4-9416-4e13-a765-7c90683f3526 {{(pid=20994) obj_load_attr /opt/stack/new/nova/nova/objects/instance.py:1038}}
Jun 06 13:14:02.577132 ubuntu-xenial-infracloud-chocolate-9158824 nova-compute[20994]: DEBUG nova.objects.instance [None req-5b3770c1-d332-4875-8933-97de8a9890b4 admin admin] Lazy-loading 'migration_context' on Instance uuid 573258a4-9416-4e13-a765-7c90683f3526 {{(pid=20994) obj_load_attr /opt/stack/new/nova/nova/objects/instance.py:1038}}
Jun 06 13:14:02.590554 ubuntu-xenial-infracloud-chocolate-9158824 nova-compute[20994]: DEBUG nova.objects.instance [None req-5b3770c1-d332-4875-8933-97de8a9890b4 admin admin] [instance: 573258a4-9416-4e13-a765-7c90683f3526] Trying to apply a migration context that does not seem to be set for this instance {{(pid=20994) apply_migration_context /opt/stack/new/nova/nova/objects/instance.py:977}}

This is wasteful as each lazy-loaded field is a round trip to the database via conductor to set the field. If self.migration_context isn't set, mutated_migration_context() should just yield and return.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/471486

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/471486
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=c3d466dd4d9739844404fdd9c4170c4a8284a4aa
Submitter: Jenkins
Branch: master

commit c3d466dd4d9739844404fdd9c4170c4a8284a4aa
Author: Matt Riedemann <email address hidden>
Date: Tue Jun 6 17:28:42 2017 -0400

    Avoid unnecessary lazy-loads in mutated_migration_context

    If the instance isn't being migrated and has the
    migration_context attribute set to None, like during a rebuild
    operation, we should detect that and exit early rather
    than lazy-load several fields (numa_topology, pci_requests,
    and pci_devices - all separately) and then just not use/need
    them.

    Change-Id: I071ab575bfd80db029d542cebfdb3d4e34227881
    Closes-Bug: #1696221

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/nova 16.0.0.0b3 development milestone.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: stable/ocata
Review: https://review.openstack.org/515581

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/515581
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=8c4db90d0398cefaac99fcef44a9e19d1bf40ad2
Submitter: Zuul
Branch: stable/ocata

commit 8c4db90d0398cefaac99fcef44a9e19d1bf40ad2
Author: Matt Riedemann <email address hidden>
Date: Tue Jun 6 17:28:42 2017 -0400

    Avoid unnecessary lazy-loads in mutated_migration_context

    If the instance isn't being migrated and has the
    migration_context attribute set to None, like during a rebuild
    operation, we should detect that and exit early rather
    than lazy-load several fields (numa_topology, pci_requests,
    and pci_devices - all separately) and then just not use/need
    them.

    Change-Id: I071ab575bfd80db029d542cebfdb3d4e34227881
    Closes-Bug: #1696221
    (cherry picked from commit c3d466dd4d9739844404fdd9c4170c4a8284a4aa)

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/nova 15.1.1 release.