From 69a67556c77d9ef2fd9110a8c8e60b834c05a3c7 Mon Sep 17 00:00:00 2001
From: Paco Xu <paco.xu@daocloud.io>
Date: Thu, 8 Aug 2024 21:02:13 +0800
Subject: [PATCH] kubelet: add warning log and events for cgroup v2 running on
 kernel < 5.8

---
 pkg/kubelet/cm/container_manager.go |  3 +++
 pkg/kubelet/events/event.go         |  1 +
 pkg/kubelet/kubelet.go              | 16 ++++++++++++++--
 pkg/util/kernel/constants.go        |  5 +++++
 4 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/pkg/kubelet/cm/container_manager.go b/pkg/kubelet/cm/container_manager.go
index 99fb69c8b98..15215cc86c8 100644
--- a/pkg/kubelet/cm/container_manager.go
+++ b/pkg/kubelet/cm/container_manager.go
@@ -46,6 +46,9 @@ import (
 const (
 	// Warning message for the users still using cgroup v1
 	CgroupV1MaintenanceModeWarning = "Cgroup v1 support is in maintenance mode, please migrate to Cgroup v2."
+
+	// Warning message for the users using cgroup v2 on kernel older than 5.8
+	CgroupV2KernelWarning = "Cgroup v2 is being used on a kernel older than 5.8, which may have limited functionality."
 )
 
 type ActivePodsFunc func() []*v1.Pod
diff --git a/pkg/kubelet/events/event.go b/pkg/kubelet/events/event.go
index a9948ba6a54..f23c68c1726 100644
--- a/pkg/kubelet/events/event.go
+++ b/pkg/kubelet/events/event.go
@@ -77,6 +77,7 @@ const (
 	FailedPrepareDynamicResources        = "FailedPrepareDynamicResources"
 	PossibleMemoryBackedVolumesOnDisk    = "PossibleMemoryBackedVolumesOnDisk"
 	CgroupV1                             = "CgroupV1"
+	CgroupV2                             = "CgroupV2"
 )
 
 // Image manager event reason list
diff --git a/pkg/kubelet/kubelet.go b/pkg/kubelet/kubelet.go
index 68d790242db..a39ee656b86 100644
--- a/pkg/kubelet/kubelet.go
+++ b/pkg/kubelet/kubelet.go
@@ -52,6 +52,7 @@ import (
 	"k8s.io/apimachinery/pkg/types"
 	utilruntime "k8s.io/apimachinery/pkg/util/runtime"
 	"k8s.io/apimachinery/pkg/util/sets"
+	"k8s.io/apimachinery/pkg/util/version"
 	"k8s.io/apimachinery/pkg/util/wait"
 	utilfeature "k8s.io/apiserver/pkg/util/feature"
 	clientset "k8s.io/client-go/kubernetes"
@@ -120,6 +121,7 @@ import (
 	"k8s.io/kubernetes/pkg/kubelet/volumemanager"
 	httpprobe "k8s.io/kubernetes/pkg/probe/http"
 	"k8s.io/kubernetes/pkg/security/apparmor"
+	utilkernel "k8s.io/kubernetes/pkg/util/kernel"
 	"k8s.io/kubernetes/pkg/util/oom"
 	"k8s.io/kubernetes/pkg/volume"
 	"k8s.io/kubernetes/pkg/volume/csi"
@@ -1645,7 +1647,7 @@ func (kl *Kubelet) Run(updates <-chan kubetypes.PodUpdate) {
 		os.Exit(1)
 	}
 
-	kl.warnCgroupV1Usage()
+	kl.cgroupVersionCheck()
 
 	// Start volume manager
 	go kl.volumeManager.Run(kl.sourcesReady, wait.NeverStop)
@@ -3067,11 +3069,21 @@ func (kl *Kubelet) UnprepareDynamicResources(pod *v1.Pod) error {
 	return kl.containerManager.UnprepareDynamicResources(pod)
 }
 
-func (kl *Kubelet) warnCgroupV1Usage() {
+func (kl *Kubelet) cgroupVersionCheck() {
 	cgroupVersion := kl.containerManager.GetNodeConfig().CgroupVersion
 	if cgroupVersion == 1 {
 		kl.recorder.Eventf(kl.nodeRef, v1.EventTypeWarning, events.CgroupV1, cm.CgroupV1MaintenanceModeWarning)
 		klog.V(2).InfoS("Warning: cgroup v1", "message", cm.CgroupV1MaintenanceModeWarning)
+	} else {
+		kernelVersion, err := utilkernel.GetVersion()
+		if err != nil {
+			klog.V(2).ErrorS(err, "Warning: cannot determine kernel version, unable to determine if cgroup v2 is supported", "message", cm.CgroupV1MaintenanceModeWarning)
+		}
+
+		if kernelVersion.LessThan(version.MustParseGeneric(utilkernel.CgroupV2KernelVersion)) {
+			kl.recorder.Eventf(kl.nodeRef, v1.EventTypeWarning, events.CgroupV2, cm.CgroupV2KernelWarning)
+			klog.V(2).InfoS("Warning: cgroup v2", "message", cm.CgroupV2KernelWarning)
+		}
 	}
 	metrics.CgroupVersion.Set(float64(cgroupVersion))
 }
diff --git a/pkg/util/kernel/constants.go b/pkg/util/kernel/constants.go
index 6775027e7a0..2bf27ac0750 100644
--- a/pkg/util/kernel/constants.go
+++ b/pkg/util/kernel/constants.go
@@ -54,3 +54,8 @@ const TmpfsNoswapSupportKernelVersion = "6.4"
 // nftables mode with by default. This is not directly related to any specific kernel
 // commit; see https://issues.k8s.io/122743#issuecomment-1893922424
 const NFTablesKubeProxyKernelVersion = "5.13"
+
+// CgroupV2KernelVersion is the minimal kernel version required for cgroup v2 support.
+// `cpu.stat` file was added to root cgroup.
+// (ref: https://github.com/torvalds/linux/commit/936f2a70f2077f64fab1dcb3eca71879e82ecd3f)
+const CgroupV2KernelVersion = "5.8"