diff --git a/nova/quota.py b/nova/quota.py
index b0e4d33997e4..29a0fc05e806 100644
--- a/nova/quota.py
+++ b/nova/quota.py
@@ -1191,8 +1191,12 @@ def _instances_cores_ram_count_legacy(context, project_id, user_id=None):
     # this filtering if there is more than one non-cell0 cell.
     # TODO(tssurya): Consider adding a scatter_gather_cells_for_project
     # variant that makes this native to nova.context.
-    cell_mappings = objects.CellMappingList.get_by_project_id(
-        context, project_id)
+    if CONF.api.instance_list_per_project_cells:
+        cell_mappings = objects.CellMappingList.get_by_project_id(
+            context, project_id)
+    else:
+        nova_context.load_cells()
+        cell_mappings = nova_context.CELLS
     results = nova_context.scatter_gather_cells(
         context, cell_mappings, nova_context.CELL_TIMEOUT,
         objects.InstanceList.get_counts, project_id, user_id=user_id)
diff --git a/nova/tests/functional/db/test_quota.py b/nova/tests/functional/db/test_quota.py
index a66ec1689959..8c2b637269e1 100644
--- a/nova/tests/functional/db/test_quota.py
+++ b/nova/tests/functional/db/test_quota.py
@@ -139,7 +139,9 @@ class QuotaTestCase(test.NoDBTestCase):
             ctxt, group, instance.user_id)
         self.assertEqual(2, count['user']['server_group_members'])
 
-    def test_instances_cores_ram_count(self):
+    @mock.patch('nova.objects.CellMappingList.get_by_project_id',
+        wraps=objects.CellMappingList.get_by_project_id)
+    def test_instances_cores_ram_count(self, mock_get_project_cell_mappings):
         ctxt = context.RequestContext('fake-user', 'fake-project')
         mapping1 = objects.CellMapping(context=ctxt,
                                        uuid=uuidutils.generate_uuid(),
@@ -197,10 +199,22 @@ class QuotaTestCase(test.NoDBTestCase):
                                          project_id='fake-project')
             im.create()
 
-        # Count instances, cores, and ram across cells
+        # Count instances, cores, and ram across cells (all cells)
         count = quota._instances_cores_ram_count(ctxt, 'fake-project',
                                                  user_id='fake-user')
+        mock_get_project_cell_mappings.assert_not_called()
+        self.assertEqual(3, count['project']['instances'])
+        self.assertEqual(10, count['project']['cores'])
+        self.assertEqual(2560, count['project']['ram'])
+        self.assertEqual(2, count['user']['instances'])
+        self.assertEqual(6, count['user']['cores'])
+        self.assertEqual(1536, count['user']['ram'])
 
+        # Count instances, cores, and ram across cells (query cell subset)
+        self.flags(instance_list_per_project_cells=True, group='api')
+        count = quota._instances_cores_ram_count(ctxt, 'fake-project',
+                                                 user_id='fake-user')
+        mock_get_project_cell_mappings.assert_called_with(ctxt, 'fake-project')
         self.assertEqual(3, count['project']['instances'])
         self.assertEqual(10, count['project']['cores'])
         self.assertEqual(2560, count['project']['ram'])