Default native threading for sch, api and metadata

This patch switches the default concurrency mode to native threading
for the services that gained native threading support in Flamingo:
nova-scheduler, nova-api, and nova-metadata.

The OS_NOVA_DISABLE_EVENTLET_PATCHING env variable still can be used to
explicitly switch the concurrency mode to eventlet by

  OS_NOVA_DISABLE_EVENTLET_PATCHING=false

We also ensure that the cover, docs, py3xx and functional tox targets
are still running with eventlet while py312-threading kept running
with native threading.

Change-Id: I86c7f31f19ca3345218171f0abfa8ddd4f8fc7ea
Signed-off-by: Balazs Gibizer <gibi@redhat.com>

doc/source/admin/concurrency.rst

@@ -21,14 +21,21 @@ up support for the native threading mode.
 Selecting concurrency mode for a service
 ----------------------------------------
 
-Nova still uses Eventlet by default, but allows switching services to native
-threading mode at service startup via setting the environment variable
-``OS_NOVA_DISABLE_EVENTLET_PATCHING=true``.
+Since nova 33.0.0 (2026.1 Gazpacho) the nova-scheduler, nova-api, and
+nova-metadata are using native threading by default. The rest of the
+services are using eventlet by default in this release. The concurrency mode
+can be configured via setting the environment variable
+``OS_NOVA_DISABLE_EVENTLET_PATCHING``. Setting that variable to ``true``
+requests the native threading mode while setting it to ``false`` requests the
+eventlet mode. If the variable is not set the above default is applied.
 
 .. note::
 
-   Since nova 32.0.0 (2025.2 Flamingo) the nova-scheduler, nova-metadata,
-   nova-api, and nova-conductor can be switched to native threading mode.
+   Since nova 32.0.0 (2025.2 Flamingo) the nova-scheduler, nova-metadata, and
+   nova-api can be switched to native threading mode.
+
+   Since nova 33.0.0 (2026.1 Gazpacho) also the nova-conductor can be switched
+   to native threading mode.
 
 
 Tunables for the native threading mode

nova/cmd/scheduler.py

@@ -16,7 +16,7 @@
 
 """Starter script for Nova Scheduler."""
 # autopep8: off
-from nova import monkey_patch; monkey_patch.patch()  # noqa
+from nova import monkey_patch; monkey_patch.patch(backend='threading')  # noqa
 # autopep8: on
 
 import sys

nova/monkey_patch.py

@@ -72,10 +72,29 @@ def _monkey_patch():
     return True
 
 
-def patch():
-    if (os.environ.get('OS_NOVA_DISABLE_EVENTLET_PATCHING', '').lower()
-        not in ('1', 'true', 'yes')):
+def patch(backend='eventlet'):
+    """Apply eventlet monkey patching according to environment.
 
+    :param backend: Defines the default backend if not explicitly set via
+        the environment. If 'eventlet', then monkey patch if environment
+        variable is not defined. If 'threading', then do not monkey patch if
+        environment variable is not defined. Any other value results in a
+        ValueError. If the environment variable is defined this parameter
+        is ignored.
+    """
+    if backend not in ('eventlet', 'threading'):
+        raise ValueError(
+            "the backend can only be 'eventlet' or 'threading'")
+
+    env = os.environ.get('OS_NOVA_DISABLE_EVENTLET_PATCHING', '').lower()
+    if env == '':
+        should_patch = (backend == 'eventlet')
+    elif env in ('1', 'true', 'yes'):
+        should_patch = False
+    else:
+        should_patch = True
+
+    if should_patch:
         if _monkey_patch():
             global MONKEY_PATCHED
             MONKEY_PATCHED = True

nova/tests/unit/test_utils.py

@@ -1691,3 +1691,17 @@ class OsloServiceBackendSelectionTestCase(test.NoDBTestCase):
             "OS_NOVA_DISABLE_EVENTLET_PATCHING set to 'true', but then the "
             "service tried to call eventlet.monkey_patch(). This is a bug.",
             str(ex))
+
+    @mock.patch('oslo_service.backend.init_backend')
+    def test_threading_selected_by_default(self, init_backend):
+        with mock.patch.dict(os.environ):
+            del os.environ["OS_NOVA_DISABLE_EVENTLET_PATCHING"]
+            monkey_patch.patch(backend='threading')
+
+        init_backend.assert_called_once_with(
+            oslo_backend.BackendType.THREADING)
+
+    def test_invalid_default_backend(self):
+        ex = self.assertRaises(ValueError, monkey_patch.patch, backend='foo')
+        self.assertEqual(
+            "the backend can only be 'eventlet' or 'threading'", str(ex))

nova/wsgi/metadata.py

@@ -12,7 +12,7 @@
 
 """WSGI application entry-point for Nova Metadata API."""
 # autopep8: off
-from nova import monkey_patch ; monkey_patch.patch()  # noqa
+from nova import monkey_patch; monkey_patch.patch(backend='threading')  # noqa
 # autopep8: on
 
 import threading

nova/wsgi/osapi_compute.py

@@ -12,7 +12,7 @@
 
 """WSGI application entry-point for Nova Compute API."""
 # autopep8: off
-from nova import monkey_patch ; monkey_patch.patch()  # noqa
+from nova import monkey_patch; monkey_patch.patch(backend='threading')  # noqa
 # autopep8: on
 
 import threading

releasenotes/notes/threading-by-default-sch-api-meta-d2534ce9c7b69d8a.yaml

@@ -0,0 +1,9 @@
+---
+features:
+  - |
+    The default concurrency mode is now switched from eventlet to native
+    threading for nova-scheduler, nova-api, and nova-metadata services.
+    The concurrency mode can still be switched back to eventlet if needed.
+    Please read the
+    `concurrency <https://docs.openstack.org/nova/latest/admin/concurrency.html>`__
+    guide for more details.

tox.ini

@@ -56,12 +56,14 @@ commands =
   env TEST_OSPROFILER=1 stestr run --combine --no-discover 'nova.tests.unit.test_profiler'
   stestr slowest
 
-[testenv:{unit,py3,py310,py311,py312}]
+[testenv:{unit,py3,py310,py311,py312,py313}]
 setenv =
   {[testenv]setenv}
 # we do not have any greenlet leaks in unit tests so enforce that
 # by making greenlet leaks a failure.
   NOVA_RAISE_ON_GREENLET_LEAK=True
+# run the test with eventlet
+  OS_NOVA_DISABLE_EVENTLET_PATCHING=False
 
 [testenv:py312-threading]
 setenv =
@@ -81,7 +83,7 @@ commands =
   stestr run {posargs} --exclude-list /tmp/exclude.txt
   stestr slowest
 
-[testenv:functional{,-py310,-py311,-py312}]
+[testenv:functional{,-py310,-py311,-py312,-py313}]
 description =
   Run functional tests.
 setenv =
@@ -89,6 +91,8 @@ setenv =
 # we do not have any greenlet leaks in functional tests so enforce that
 # by making greenlet leaks a failure.
   NOVA_RAISE_ON_GREENLET_LEAK=True
+# run the test with eventlet
+  OS_NOVA_DISABLE_EVENTLET_PATCHING=False
 # As nova functional tests import the PlacementFixture from the placement
 # repository these tests are, by default, set up to run with openstack-placement
 # from pypi. In the gate, Zuul will use the installed version of placement (stable
@@ -208,6 +212,8 @@ commands =
 setenv =
   {[testenv]setenv}
   PYTHON=coverage run --source nova --parallel-mode
+# run the test with eventlet
+  OS_NOVA_DISABLE_EVENTLET_PATCHING=False
 extras =
 commands =
   coverage erase
@@ -244,6 +250,9 @@ commands =
   sphinx-build -W --keep-going -b html -j auto doc/source doc/build/html
   # Test the redirects. This must run after the main docs build
   whereto doc/build/html/.htaccess doc/test/redirect-tests.txt
+setenv =
+  OS_NOVA_DISABLE_EVENTLET_PATCHING=False
+
 
 [testenv:pdf-docs]
 description =