WIP: Implement system-reserved-compressible

[ngopalak-redhat]

TODO: Before Review

• Complete upgrade testing

What I did

This PR enables system-reserved-compressible enforcement by default for all new OpenShift 4.21+ clusters to allow better CPU allocation for system reserved processes through cgroup-based enforcement.

Template Changes:

• Added systemReservedCgroup: /system.slice to default kubelet configuration for all node types (master, worker, arbiter)
• Added system-reserved-compressible to enforceNodeAllocatable alongside pods in kubelet template files

Performance Profile Compatibility:
The kubelet cannot simultaneously enforce both systemReservedCgroup and --reserved-cpus (used by Performance Profiles in the Node Tuning Operator). To resolve this conflict, I added logic in the Kubelet Config Controller (pkg/controller/kubelet-config/helpers.go) to:

• Detect when reservedSystemCPUs (--reserved-cpus) is set
• Automatically clear systemReservedCgroup when reservedSystemCPUs is detected
• Set enforceNodeAllocatable to ["pods"] only in this scenario
• Preserve existing Performance Profile behavior without requiring any operator changes

This approach leverages the fact that --reserved-cpus already supersedes system-reserved, making systemReservedCgroup enforcement redundant in PerformanceProfile scenarios.

Validation:

• Added validation to ensure systemReservedCgroup matches systemCgroups when both are user-specified

How to verify it

For New OCP 4.21+ Clusters:

1. Deploy a new OCP 4.21+ cluster
2. SSH into a node and verify kubelet configuration:
   cat /etc/kubernetes/kubelet.conf | grep -A2 systemReservedCgroup
   cat /etc/kubernetes/kubelet.conf | grep -A3 enforceNodeAllocatable
3. Verify the output shows:
   systemReservedCgroup: /system.slice
   enforceNodeAllocatable:

• pods
• system-reserved-compressible

For Clusters with Performance Profiles:

1. Create a Performance Profile with reservedSystemCPUs set (via Node Tuning Operator)
2. Wait for the MachineConfig to be applied and nodes to reboot
3. SSH into the affected node and check kubelet configuration:
   cat /etc/kubernetes/kubelet.conf | grep systemReservedCgroup
   cat /etc/kubernetes/kubelet.conf | grep enforceNodeAllocatable
4. Verify that:
   - systemReservedCgroup is NOT present (empty/cleared)
   - enforceNodeAllocatable only contains ["pods"]
   - Kubelet starts successfully without errors
5. Check kubelet logs to confirm no conflicts:
   journalctl -u kubelet | grep -i "system-reserved|reserved-cpus"

For OCP 4.20 to 4.21 Upgrades:

1. Verify that the migration MachineConfig from PR WIP : [release-4.20] kubelet-config compressible patch #5412 is present and preserves old behavior
2. Confirm no unexpected node reboots occur during upgrade

Description for the changelog

Enable system-reserved-compressible enforcement by default in new OCP 4.21+ clusters. The kubelet now enforces CPU limits on system daemons via systemReservedCgroup (/system.slice), improving CPU allocation for system reserved processes on nodes with high CPU counts. Automatically disables systemReservedCgroup enforcement when Performance Profiles with reserved-cpus are used to prevent conflicts. Existing OCP 4.20 clusters upgrading to 4.21+ will preserve their current behavior via migration MachineConfig.

---

Related:

• JIRA: https://issues.redhat.com/browse/OCPNODE-3201
• Related: https://issues.redhat.com/browse/OCPNODE-3200
• Migration PR: WIP : [release-4.20] kubelet-config compressible patch #5412

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