cpu: make governors to be optional

Change-Id: Ifb7d001cfdb95b1b0aa29f45c0ef71c0673e1760
Closes-Bug: #2023018

doc/source/admin/cpu-topologies.rst

@@ -772,6 +772,11 @@ while there could be other tools outside Nova to manage the governor, like
 tuned. That being said, we also provide a way to automatically change the
 governors on the fly, as explained below.
 
+.. important::
+   Some OS platforms don't support `cpufreq` resources in sysfs, so the
+   ``governor`` strategy could be not available. Please verify if your OS
+   supports scaling govenors before modifying the configuration option.
+
 If the strategy is set to ``governor``, a couple of config options are provided
 to define which exact CPU govenor to use for each of the up and down states :
 

nova/tests/fixtures/filesystem.py

@@ -39,10 +39,14 @@ class TempFileSystemFixture(fixtures.Fixture):
 
 
 class SysFileSystemFixture(TempFileSystemFixture):
-    """Creates a fake /sys filesystem"""
+    def __init__(self, cpus_supported=None, cpufreq_enabled=True):
+        """Instantiates a fake sysfs.
 
-    def __init__(self, cpus_supported=None):
+        :param cpus_supported: number of devices/system/cpu (default: 10)
+        :param cpufreq_enabled: cpufreq subdir created (default: True)
+        """
         self.cpus_supported = cpus_supported or 10
+        self.cpufreq_enabled = cpufreq_enabled
 
     def _setUp(self):
         super()._setUp()
@@ -73,6 +77,9 @@ class SysFileSystemFixture(TempFileSystemFixture):
 
         for cpu_nr in range(self.cpus_supported):
             cpu_dir = os.path.join(self.cpu_path_mock % {'core': cpu_nr})
+            os.makedirs(cpu_dir)
+            filesystem.write_sys(os.path.join(cpu_dir, 'online'), data='1')
+            if self.cpufreq_enabled:
                 os.makedirs(os.path.join(cpu_dir, 'cpufreq'))
                 filesystem.write_sys(
                     os.path.join(cpu_dir, 'cpufreq/scaling_governor'),

nova/tests/functional/libvirt/test_power_manage.py

@@ -221,6 +221,39 @@ class PowerManagementTestsGovernor(PowerManagementTestsBase):
                           self.restart_compute_service, hostname='compute1')
 
 
+class PowerManagementTestsGovernorNotSupported(PowerManagementTestsBase):
+    """Test suite for OS without governor support usage (same 10-core host)"""
+
+    def setUp(self):
+        super(PowerManagementTestsGovernorNotSupported, self).setUp()
+
+        self.useFixture(nova_fixtures.SysFileSystemFixture(
+            cpufreq_enabled=False))
+
+        # Definining the CPUs to be pinned.
+        self.flags(cpu_dedicated_set='1-9', cpu_shared_set=None,
+                   group='compute')
+        self.flags(vcpu_pin_set=None)
+        self.flags(cpu_power_management=True, group='libvirt')
+        self.flags(cpu_power_management_strategy='cpu_state', group='libvirt')
+
+        self.flags(allow_resize_to_same_host=True)
+        self.host_info = fakelibvirt.HostInfo(cpu_nodes=1, cpu_sockets=1,
+                                              cpu_cores=5, cpu_threads=2)
+
+    def test_enabling_governor_strategy_fails(self):
+        self.flags(cpu_power_management_strategy='governor', group='libvirt')
+        self.assertRaises(exception.FileNotFound, self.start_compute,
+                          host_info=self.host_info, hostname='compute1')
+
+    def test_enabling_cpu_state_strategy_works(self):
+        self.flags(cpu_power_management_strategy='cpu_state', group='libvirt')
+        self.compute1 = self.start_compute(host_info=self.host_info,
+                                           hostname='compute1')
+        cpu_dedicated_set = hardware.get_cpu_dedicated_set()
+        self._assert_cpu_set_state(cpu_dedicated_set, expected='offline')
+
+
 class PowerManagementMixedInstances(PowerManagementTestsBase):
     """Test suite for a single host with 6 dedicated cores, 3 shared and one
     OS-restricted.

nova/tests/unit/virt/libvirt/cpu/test_api.py

@@ -57,6 +57,12 @@ class TestAPI(test.NoDBTestCase):
         self.assertEqual('fake_governor', self.core_1.governor)
         mock_get_governor.assert_called_once_with(self.core_1.ident)
 
+    @mock.patch.object(core, 'get_governor')
+    def test_governor_optional(self, mock_get_governor):
+        mock_get_governor.side_effect = exception.FileNotFound(file_path='foo')
+        self.assertIsNone(self.core_1.governor)
+        mock_get_governor.assert_called_once_with(self.core_1.ident)
+
     @mock.patch.object(core, 'set_governor')
     def test_set_governor_low(self, mock_set_governor):
         self.flags(cpu_power_governor_low='fake_low_gov', group='libvirt')

nova/virt/libvirt/cpu/api.py

@@ -11,6 +11,7 @@
 #    under the License.
 
 from dataclasses import dataclass
+import typing as ty
 
 from oslo_log import log as logging
 
@@ -59,8 +60,13 @@ class Core:
         return str(self.ident)
 
     @property
-    def governor(self) -> str:
+    def governor(self) -> ty.Optional[str]:
+        try:
             return core.get_governor(self.ident)
+        # NOTE(sbauza): cpufreq/scaling_governor is not enabled for some OS
+        # platforms.
+        except exception.FileNotFound:
+            return None
 
     def set_high_governor(self) -> None:
         core.set_governor(self.ident, CONF.libvirt.cpu_power_governor_high)

releasenotes/notes/bug-2023018-0f93ca1f679ce259.yaml

@@ -0,0 +1,9 @@
+---
+fixes:
+  - |
+    Some OS platforms don't provide by default cpufreq resources in sysfs, so
+    they don't have CPU scaling governors. That's why we should let the governor
+    strategy to be optional for `CPU power management`_.
+
+    .. _CPU power management: https://docs.openstack.org/nova/latest/admin/cpu-topologies.html#configuring-cpu-power-management-for-dedicated-cores
+