cpu: interfaces for managing state and governor

This is the first stage of the power management series.
In order to be able to switch the CPU state or change the
governor, we need a framework to access sysfs.

As some bits can be reused, let's create a nova.filesystem helper module
that will define read-write mechanisms for accessing sysfs-specific commands.

Partially-Implements: blueprint libvirt-cpu-state-mgmt
Change-Id: Icb913ed9be8d508de35e755a9c650ba25e45aca2

3 files changed, 156 insertions, 0 deletions

diff --git a/nova/virt/libvirt/cpu/__init__.py b/nova/virt/libvirt/cpu/__init__.py
new file mode 100644
index 0000000000..962c9469a0
--- /dev/null
+++ b/nova/virt/libvirt/cpu/__init__.py
@@ -0,0 +1,16 @@
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from nova.virt.libvirt.cpu import api
+
+
+Core = api.Core
diff --git a/nova/virt/libvirt/cpu/api.py b/nova/virt/libvirt/cpu/api.py
new file mode 100644
index 0000000000..e0b0a277d1
--- /dev/null
+++ b/nova/virt/libvirt/cpu/api.py
@@ -0,0 +1,62 @@
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from dataclasses import dataclass
+
+from oslo_log import log as logging
+
+import nova.conf
+from nova.virt.libvirt.cpu import core
+
+LOG = logging.getLogger(__name__)
+
+CONF = nova.conf.CONF
+
+
+@dataclass
+class Core:
+    """Class to model a CPU core as reported by sysfs.
+
+    It may be a physical CPU core or a hardware thread on a shared CPU core
+    depending on if the system supports SMT.
+    """
+
+    # NOTE(sbauza): ident is a mandatory field.
+    # The CPU core id/number
+    ident: int
+
+    @property
+    def online(self) -> bool:
+        return core.get_online(self.ident)
+
+    @online.setter
+    def online(self, state: bool) -> None:
+        if state:
+            core.set_online(self.ident)
+        else:
+            core.set_offline(self.ident)
+
+    def __hash__(self):
+        return hash(self.ident)
+
+    def __eq__(self, other):
+        return self.ident == other.ident
+
+    @property
+    def governor(self) -> str:
+        return core.get_governor(self.ident)
+
+    def set_high_governor(self) -> None:
+        core.set_governor(self.ident, CONF.libvirt.cpu_power_governor_high)
+
+    def set_low_governor(self) -> None:
+        core.set_governor(self.ident, CONF.libvirt.cpu_power_governor_low)
diff --git a/nova/virt/libvirt/cpu/core.py b/nova/virt/libvirt/cpu/core.py
new file mode 100644
index 0000000000..782f028fee
--- /dev/null
+++ b/nova/virt/libvirt/cpu/core.py
@@ -0,0 +1,78 @@
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+import os
+import typing as ty
+
+from oslo_log import log as logging
+
+from nova import exception
+from nova import filesystem
+import nova.privsep
+from nova.virt import hardware
+
+LOG = logging.getLogger(__name__)
+
+AVAILABLE_PATH = '/sys/devices/system/cpu/present'
+
+CPU_PATH_TEMPLATE = '/sys/devices/system/cpu/cpu%(core)s'
+
+
+def get_available_cores() -> ty.Set[int]:
+    cores = filesystem.read_sys(AVAILABLE_PATH)
+    return hardware.parse_cpu_spec(cores) if cores else set()
+
+
+def exists(core: int) -> bool:
+    return core in get_available_cores()
+
+
+def gen_cpu_path(core: int) -> str:
+    if not exists(core):
+        LOG.warning('Unable to access CPU: %s', core)
+        raise ValueError('CPU: %(core)s does not exist', core)
+    return CPU_PATH_TEMPLATE % {'core': core}
+
+
+def get_online(core: int) -> bool:
+    try:
+        online = filesystem.read_sys(
+            os.path.join(gen_cpu_path(core), 'online')).strip()
+    except exception.FileNotFound:
+        # The online file may not exist if we haven't written it yet.
+        # By default, this means that the CPU is online.
+        online = '1'
+    return online == '1'
+
+
+@nova.privsep.sys_admin_pctxt.entrypoint
+def set_online(core: int) -> bool:
+    filesystem.write_sys(os.path.join(gen_cpu_path(core), 'online'), data='1')
+    return get_online(core)
+
+
+def set_offline(core: int) -> bool:
+    filesystem.write_sys(os.path.join(gen_cpu_path(core), 'online'), data='0')
+    return not get_online(core)
+
+
+def get_governor(core: int) -> str:
+    return filesystem.read_sys(
+        os.path.join(gen_cpu_path(core), 'cpufreq/scaling_governor')).strip()
+
+
+@nova.privsep.sys_admin_pctxt.entrypoint
+def set_governor(core: int, governor: str) -> str:
+    filesystem.write_sys(
+        os.path.join(gen_cpu_path(core), 'cpufreq/scaling_governor'),
+        data=governor)
+    return get_governor(core)