[pwmgmt]ignore missin governor when cpu_state used

When cpu_state power management strategy is requested nova-compute
should not fail to start if there is no cpufreq governor is supported by
the host.

Closes-Bug: #2045966
Change-Id: Ice2fa47bdab320a7e472fbb4767761448d176bad

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

@@ -243,3 +243,42 @@ class TestAPI(test.NoDBTestCase):
         api.validate_all_dedicated_cpus()
         # no assert we want to make sure the validation won't raise if
         # no dedicated cpus are configured
+
+    @mock.patch.object(core, 'get_governor')
+    @mock.patch.object(core, 'get_online')
+    def test_validate_all_dedicated_cpus_for_cpu_state_no_governor_ignored(
+        self, mock_get_online, mock_get_governor
+    ):
+        self.flags(cpu_power_management=True, group='libvirt')
+        self.flags(cpu_dedicated_set='0-2', group='compute')
+        self.flags(cpu_power_management_strategy='cpu_state', group='libvirt')
+
+        mock_get_online.return_value = True
+        mock_get_governor.side_effect = FileNotFoundError(
+            "File /sys/devices/system/cpu/cpu1/cpufreq/scaling_governor "
+            "could not be found.")
+
+        api.validate_all_dedicated_cpus()
+
+        self.assertEqual(2, len(mock_get_governor.mock_calls))
+
+    @mock.patch.object(core, 'get_governor')
+    @mock.patch.object(core, 'get_online')
+    def test_validate_all_dedicated_cpus_for_governor_error(
+        self, mock_get_online, mock_get_governor
+    ):
+        self.flags(cpu_power_management=True, group='libvirt')
+        self.flags(cpu_dedicated_set='0-2', group='compute')
+        self.flags(cpu_power_management_strategy='governor', group='libvirt')
+
+        mock_get_online.return_value = True
+        mock_get_governor.side_effect = FileNotFoundError(
+            "File /sys/devices/system/cpu/cpu1/cpufreq/scaling_governor "
+            "could not be found.")
+
+        ex = self.assertRaises(
+            exception.InvalidConfiguration, api.validate_all_dedicated_cpus)
+        self.assertIn(
+            "[libvirt]cpu_power_management_strategy is 'governor', "
+            "but the host OS does not support governors for CPU0",
+            str(ex))

nova/virt/libvirt/cpu/api.py

@@ -143,7 +143,24 @@ def validate_all_dedicated_cpus() -> None:
             continue
         pcpu = Core(pcpu)
         # we need to collect the governors strategy and the CPU states
-        governors.add(pcpu.governor)
+        try:
+            governors.add(pcpu.governor)
+        except FileNotFoundError as e:
+            # NOTE(gibi): When
+            # /sys/devices/system/cpu/cpuX/cpufreq/scaling_governor does
+            # not exist it means the host OS does not support any governors.
+            # If cpu_state strategy is requested we can ignore this as
+            # governors will not be used but if governor strategy is requested
+            # we need to report an error and stop as the host is not properly
+            # configured
+            if CONF.libvirt.cpu_power_management_strategy == 'governor':
+                msg = _(
+                    "[libvirt]cpu_power_management_strategy is 'governor', "
+                    "but the host OS does not support governors for CPU%d"
+                    % pcpu.ident
+                )
+                raise exception.InvalidConfiguration(msg) from e
+
         cpu_states.add(pcpu.online)
     if CONF.libvirt.cpu_power_management_strategy == 'cpu_state':
         # all the cores need to have the same governor strategy