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Enabling Linux control group version 2 (cgroup v2)

You can enable Linux control group version 2 (cgroup v2) in your cluster by editing the node.config object. Enabling cgroup v2 in OpenShift Container Platform disables all cgroups version 1 controllers and hierarchies in your cluster. cgroup v1 is enabled by default.

cgroup v2 is the current version of the Linux cgroup API. cgroup v2 offers several improvements over cgroup v1, including a unified hierarchy, safer sub-tree delegation, new features such as Pressure Stall Information, and enhanced resource management and isolation. However, cgroup v2 has different CPU, memory, and I/O management characteristics than cgroup v1. Therefore, some workloads might experience slight differences in memory or CPU usage on clusters that run cgroup v2.

  • If you run third-party monitoring and security agents that depend on the cgroup file system, update the agents to a version that supports cgroup v2.

  • If you have configured cgroup v2 and run cAdvisor as a stand-alone daemon set for monitoring pods and containers, update cAdvisor to v0.43.0 or later.

  • If you deploy Java applications, use versions that fully support cgroup v2, such as the following packages:

    • OpenJDK / HotSpot: jdk8u372, 11.0.16, 15 and later

    • IBM Semeru Runtimes: jdk8u345-b01, 11.0.16.0, 17.0.4.0, 18.0.2.0 and later

    • IBM SDK Java Technology Edition Version (IBM Java): 8.0.7.15 and later

OpenShift Container Platform cgroups version 2 support is a Technology Preview feature only. Technology Preview features are not supported with Red Hat production service level agreements (SLAs) and might not be functionally complete. Red Hat does not recommend using them in production. These features provide early access to upcoming product features, enabling customers to test functionality and provide feedback during the development process.

For more information about the support scope of Red Hat Technology Preview features, see Technology Preview Features Support Scope.

Configuring Linux cgroup v2

You enable cgroup v2 by editing the node.config object.

Currently, disabling CPU load balancing is not supported by cgroup v2. As a result, you might not get the desired behavior from performance profiles if you have cgroup v2 enabled. Enabling cgroup v2 is not recommended if you are using performance profiles.
Prerequisites

  • You have a running OpenShift Container Platform cluster that uses version 4.12 or later.

  • You are logged in to the cluster as a user with administrative privileges.

  • You have enabled the TechPreviewNoUpgrade feature set by using the feature gates.

Procedure

  1. Enable cgroup v2 on nodes:

    1. Edit the node.config object:

      $ oc edit nodes.config/cluster

    2. Add spec.cgroupMode: "v2":

      Example node.config object
      apiVersion: config.openshift.io/v1
kind: Node
metadata:
  annotations:
    include.release.openshift.io/ibm-cloud-managed: "true"
    include.release.openshift.io/self-managed-high-availability: "true"
    include.release.openshift.io/single-node-developer: "true"
    release.openshift.io/create-only: "true"
  creationTimestamp: "2022-07-08T16:02:51Z"
  generation: 1
  name: cluster
  ownerReferences:
  - apiVersion: config.openshift.io/v1
    kind: ClusterVersion
    name: version
    uid: 36282574-bf9f-409e-a6cd-3032939293eb
  resourceVersion: "1865"
  uid: 0c0f7a4c-4307-4187-b591-6155695ac85b
spec:
  cgroupMode: "v2" (1)
...
      1 Enables cgroup v2.
Verification

  1. Check the machine configs to see that the new machine configs were added:

    $ oc get mc
    Example output
    NAME                                               GENERATEDBYCONTROLLER                      IGNITIONVERSION   AGE
00-master                                          52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
00-worker                                          52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
01-master-container-runtime                        52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
01-master-kubelet                                  52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
01-worker-container-runtime                        52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
01-worker-kubelet                                  52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
97-master-generated-kubelet                        52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0              3m (1)
99-worker-generated-kubelet                        52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0              3m
99-master-generated-registries                     52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
99-master-ssh                                                                                 3.2.0             40m
99-worker-generated-registries                     52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
99-worker-ssh                                                                                 3.2.0             40m
rendered-master-23e785de7587df95a4b517e0647e5ab7   52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
rendered-worker-5d596d9293ca3ea80c896a1191735bb1   52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
worker-enable-cgroups-v2                                                                      3.2.0             10s
    1 New machine configs are created, as expected.

  2. Check that the new kernelArguments were added to the new machine configs:

    $ oc describe mc <name>
    Example output
    apiVersion: machineconfiguration.openshift.io/v1
kind: MachineConfig
metadata:
  labels:
    machineconfiguration.openshift.io/role: worker
  name: 05-worker-kernelarg-selinuxpermissive
spec:
  kernelArguments:
  - systemd_unified_cgroup_hierarchy=1 (1)
  - cgroup_no_v1="all" (2)
  - psi=1 (3)
    1 Enables cgroup v2 in systemd.
    2 Disables cgroups v1.
    3 Enables the Linux Pressure Stall Information (PSI) feature.

  3. Check the nodes to see that scheduling on the nodes is disabled. This indicates that the change is being applied:

    $ oc get nodes
    Example output
    NAME                                       STATUS                     ROLES    AGE   VERSION
ci-ln-fm1qnwt-72292-99kt6-master-0         Ready                      master   58m   v1.25.0
ci-ln-fm1qnwt-72292-99kt6-master-1         Ready                      master   58m   v1.25.0
ci-ln-fm1qnwt-72292-99kt6-master-2         Ready                      master   58m   v1.25.0
ci-ln-fm1qnwt-72292-99kt6-worker-a-h5gt4   Ready,SchedulingDisabled   worker   48m   v1.25.0
ci-ln-fm1qnwt-72292-99kt6-worker-b-7vtmd   Ready                      worker   48m   v1.25.0
ci-ln-fm1qnwt-72292-99kt6-worker-c-rhzkv   Ready                      worker   48m   v1.25.0

  4. After a node returns to the Ready state, start a debug session for that node:

    $ oc debug node/<node_name>

  5. Set /host as the root directory within the debug shell:

    sh-4.4# chroot /host

  6. Check that the sys/fs/cgroup/cgroup2fs file is present on your nodes. This file is created by cgroup v2:

    $ stat -c %T -f /sys/fs/cgroup
    Example output
    cgroup2fs
Additional resources