Update cgroups enhancement proposal to support the removal of cgroupv1

Signed-off-by: Sai Ramesh Vanka <[email protected]>

enhancements/machine-config/mco-cgroupsv2-support.md

@@ -2,6 +2,7 @@
 title: Control Group v2 Enablement
 authors:
   - "@rphillips"
+  - "@sairameshv"
 reviewers:
   - "@mrunalp"
   - "@kikisdeliveryservice"
@@ -10,11 +11,12 @@ reviewers:
   - "@cgwalters"
 approvers:
   - "@mrunalp"
-  - "@sinnykumari"
+  - "@yuqi-zhang"
 api-approvers:
+  - "@deads2k"
   - "@sttts"
 creation-date: 2021-10-19
-last-updated: 2021-10-20
+last-updated: 2025-02-05
 status: implementable
 ---
 
@@ -40,12 +42,10 @@ status: implementable
 
 ## Summary
 
-Control Group v2 (cgroup v2) enablement in Kubernetes has progressed to beta
+Control Group v2 (cgroup v2) enablement in Kubernetes has progressed to stable
 [upstream](https://github.com/kubernetes/enhancements/tree/master/keps/sig-node/2254-cgroup-v2).
-The underlying runtime (cri-o) and supporting subsystems are now ready for
-customers to begin their own testing with it. Not all workloads will be
-compatible with cgroup v2, so it will *not* be enabled by default within
-OpenShift at this time.
+cgroup v2 is enabled by default in the freshly installed Openshift clusters >= 4.14.
+Control Group v1 is a deprecated feature from OCP 4.16 and the support is intended to be removed from OCP >= 4.19 as the related version RHEL doesn't support it
 
 Note: This enhancement is focusing on `pure` mode cgroup v2. Mixed mode environments
 may behave differently (metrics, vpa, hpa, etc) since cgroup v1 is not
@@ -70,6 +70,8 @@ Some features of cgroup v2 include:
 
 - [ ] Enable cgroup v2 within the Openshift API
 - [ ] Add kernel flags to MCO to enable cgroup v2 on nodes
+- [ ] Remove support to configure cgroup v1 in OCP clusters
+- [ ] Block the upgrades of the clusters using cgroup v1 until migrated to cgroup v2
 
 ### Non-Goals
 
@@ -81,12 +83,9 @@ to gather data from.
 
 ## Proposal
 
-The option to enable cgroup v2 will have to reside in a centralized location.
-The [OpenShift Infrastructure config
-object](https://github.com/openshift/api/blob/master/config/v1/types_infrastructure.go#L28)
-contains information describing how a cluster functions including cloud config  
-and platform specification for each cloud. Setting the cgroup mode is an
-infrastructure setting.
+The option to enable cgroup v2 resides in a centralized location i.e. [OpenShift Node config
+object](https://github.com/openshift/api/blob/master/config/v1/types_node.go)
+Set the upgrade ability of the MCO cluster operator to `false` if a cluster is on cgroup v1
 
 ### API Extensions
 
@@ -121,12 +120,11 @@ type NodeSpec struct {
 
 ### Operational Aspects of API Extensions
 
-Once the previous API is defined, the MCO will read the configured object and
-set the appropriate kernel options (on bootstrap). The MCO will report an error
+MCO reads the configured object and
+sets the appropriate kernel options (on bootstrap). The MCO will report an error
 if a user tries to modify/add cgroup related kargs within a MachineConfig.
-
-
-The following kernel command line arguments would be set when `CgroupMode_v2` is enabled:
+MCO also reports error if a user tries to set the `CgroupMode` to `CgroupMode_V1`
+The following kernel command line arguments would be observed on the machine config pools by default and also when `CgroupMode_v2` is enabled:
 ```yaml
   kernelArguments:
     - systemd.unified_cgroup_hierarchy=1
@@ -201,19 +199,31 @@ The following jobs will be run against cgroup v2 periodically and with a minimum
 
 ### Upgrade / Downgrade Strategy
 
-Downgrading a cluster to an OpenShift version not containing cgroup v2 support
+- Downgrading a cluster to an OpenShift version not containing cgroup v2 support
 is unsupported.
+- Upgrading a cluster which is on cgroup v1 to a version >= 4.19 is blocked until it is migrated to cgroup v2
 
 ### Version Skew Strategy
 
 A cluster installed with cgroup v2 will abide by the usual skew upgrade path.
 
 #### Removing a deprecated feature
 
-N/A
+cgroup v1 support would be removed from the future/associated versions of RHEL and hence the setting of `CgroupMode_V1` has to be removed from OCP clusters(>= 4.19)
 
 ## Implementation History
-
+Following code change inside the MCO [operator](https://github.com/openshift/machine-config-operator/blob/master/pkg/operator/status.go#L265)'s `pkg/operator/status.go` sets the ClusterOperator's `Upgradeable` status to `False` if the cluster is found to be using `CgroupMode_V1`
+```go
+configNode, err := optr.configClient.ConfigV1().Nodes().Get(context.Background(), ctrlcommon.ClusterNodeInstanceName, metav1.GetOptions{})
+	if err != nil {
+		return err
+	}
+	if configNode.Spec.CgroupMode == configv1.CgroupModeV1 {
+		coStatusCondition.Status = configv1.ConditionFalse
+		coStatusCondition.Reason = "ClusterOnCgroupV1"
+		coStatusCondition.Message = "Cluster is using cgroup v1, consider migrating to cgroup v2 by updating `CgroupMode` in the `nodes.config.openshift.io` object before upgrading"
+	}
+```
 ## Alternatives
 
 ## Drawbacks