Enable cpus when an instance is spawning

By this patch, we now automatically power down or up cores
when an instance is either stopped or started.

Also, by default, we now powersave or offline dedicated cores when
starting the compute service.

Implements: blueprint libvirt-cpu-state-mgmt
Change-Id: Id645fd1ba909683af903f3b8f11c7f06db3401cb

1 files changed, 18 insertions, 0 deletions

diff --git a/releasenotes/notes/bp-libvirt-cpu-state-mgmt-fbc9c1f9f473003c.yaml b/releasenotes/notes/bp-libvirt-cpu-state-mgmt-fbc9c1f9f473003c.yaml
new file mode 100644
index 0000000000..95422fce67
--- /dev/null
+++ b/releasenotes/notes/bp-libvirt-cpu-state-mgmt-fbc9c1f9f473003c.yaml
@@ -0,0 +1,18 @@
+---
+features:
+  - |
+    This is now possible to configure nova-compute services using libvirt driver
+    by setting ``[libvirt]cpu_power_management`` to ``True`` in order to let the
+    service to powering down or up physical CPUs depending on whether those CPUs
+    are pinned or not to instances. In order on to support this feature, the
+    compute service needs to be set with ``[compute]cpu_dedicated_set``. If so,
+    all the related CPUs will be powering down until they are used by an
+    instance where the related pinned CPU will be powering up just before
+    starting the guest. If ``[compute]cpu_dedicated_set`` isn't set, then the
+    compute service will refuse to start.
+    By default the power strategy will offline CPUs when powering down and
+    online the CPUs on powering up but another strategy is possible by using
+    ``[libvirt]cpu_power_management_strategy=governor`` which will rather modify
+    the related CPU governor using ``[libvirt]cpu_power_governor_low`` and
+    ``[libvirt]cpu_power_governor_high`` configuration values (respective
+    defaults being  ``powersave`` and ``performance``)