Run nova-compute in native threading mode

Previous patches removed direct eventlet usage from nova-compute so
now we can run it with native threading as well. This patch documents
the possibility and switches both nova-compute processes to native
threading mode in the nova-next job.

Change-Id: I7bb29c627326892d1cf628bbf57efbaedda12f1a
Signed-off-by: Balazs Gibizer <gibi@redhat.com>

.zuul.yaml

@@ -434,6 +434,11 @@
       devstack_local_conf:
         post-config:
           $NOVA_CPU_CONF:
+            DEFAULT:
+              # Added in Gazpacho. Ensures that thread statistics is logged
+              # each time a task is submitted to the executor so that we
+              # can troubleshoot hanging threads easier.
+              thread_pool_statistic_period: 0
             libvirt:
               # Increase the number of PCIe ports per instance given the q35
               # machine type attaches more devices by default than pc
@@ -519,6 +524,7 @@
         'SYSTEMD_ENV_VARS["n-api-meta"]': OS_NOVA_DISABLE_EVENTLET_PATCHING=true
         'SYSTEMD_ENV_VARS["n-cond-cell1"]': OS_NOVA_DISABLE_EVENTLET_PATCHING=true
         'SYSTEMD_ENV_VARS["n-super-cond"]': OS_NOVA_DISABLE_EVENTLET_PATCHING=true
+        'SYSTEMD_ENV_VARS["n-cpu"]': OS_NOVA_DISABLE_EVENTLET_PATCHING=true
       devstack_services:
         # Disable OVN services
         br-ex-tcpdump: false
@@ -555,6 +561,7 @@
           FORCE_CONFIG_DRIVE: True
           # Added in Yoga.
           NOVNC_FROM_PACKAGE: False
+          'SYSTEMD_ENV_VARS["n-cpu"]': OS_NOVA_DISABLE_EVENTLET_PATCHING=true
         devstack_services:
           # Disable OVN services
           br-ex-tcpdump: false

doc/source/admin/concurrency.rst

@@ -34,8 +34,8 @@ eventlet mode. If the variable is not set the above default is applied.
    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.
+   Since nova 33.0.0 (2026.1 Gazpacho) the nova-conductor and nova-compute
+   can also be switched to native threading mode.
 
    Since nova 33.0.0 (2026.1. Gazpacho) the nova-scheduler, nova-metadata, and
    nova-api using native threading mode by default but still can be switched

releasenotes/notes/threaded-nova-compute-9d28661295e09d74.yaml

@@ -0,0 +1,9 @@
+---
+features:
+  - |
+    The nova-compute services now can be run in native threading
+    mode instead of with eventlet. This is an experimental feature that is
+    disabled by default. Please test the native threading mode in
+    pre-production before enabling it in production. Please read the
+    `concurrency <https://docs.openstack.org/nova/latest/admin/concurrency.html>`__
+    guide for more details.