Avoid unnecessary joins in delete_resource_provider

If cascade=True, we're getting all of the instances on the
compute node just to use the uuid, which will by default
join on the info_cache and security_groups for the instances.
This is a simple optimization to avoid those unnecessary joins.

A TODO is left to further optimize this with a new InstanceList
query method to just get the instance uuids on a given host/node
combo, but that requires an RPC change which we can't backport.

Change-Id: Ie121210456a240c257979d3269db115ddae2d23b
Related-Bug: #1811726
Related-Bug: #1756179

nova/scheduler/client/report.py

@@ -2169,8 +2169,12 @@ class SchedulerReportClient(object):
             # Delete any allocations for this resource provider.
             # Since allocations are by consumer, we get the consumers on this
             # host, which are its instances.
-            instances = objects.InstanceList.get_by_host_and_node(context,
-                    host, nodename)
+            # TODO(mriedem): Optimize this up by adding an
+            # InstanceList.get_uuids_by_host_and_node method.
+            # Pass expected_attrs=[] to avoid joining on extra columns we
+            # don't use.
+            instances = objects.InstanceList.get_by_host_and_node(
+                context, host, nodename, expected_attrs=[])
             for instance in instances:
                 self.delete_allocation_for_instance(context, instance.uuid)
         try:

nova/tests/unit/scheduler/client/test_report.py

@@ -3201,6 +3201,8 @@ class TestAllocations(SchedulerReportClientTestCase):
         resp_mock = mock.Mock(status_code=204)
         mock_delete.return_value = resp_mock
         self.client.delete_resource_provider(self.context, cn, cascade=True)
+        mock_by_host.assert_called_once_with(
+            self.context, cn.host, cn.hypervisor_hostname, expected_attrs=[])
         self.assertEqual(2, mock_del_alloc.call_count)
         exp_url = "/resource_providers/%s" % uuids.cn
         mock_delete.assert_called_once_with(