check root cpu.stat instead of kernel version for cgroup v2

pkg/kubelet/cm/container_manager.go

@@ -45,10 +45,13 @@ import (
 
 const (
 	// Warning message for the users still using cgroup v1
-	CgroupV1MaintenanceModeWarning = "Cgroup v1 support is in maintenance mode, please migrate to Cgroup v2."
+	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."
+	// Warning message for the users using cgroup v2 on kernel doesn't support root `cpu.stat`.
+	// `cpu.stat` was added to root cgroup in kernel 5.8.
+	// (ref: https://github.com/torvalds/linux/commit/936f2a70f2077f64fab1dcb3eca71879e82ecd3f)
+	CgroupV2KernelWarning = "cgroup v2 is being used on a kernel, which doesn't support root `cpu.stat`." +
+		"Kubelet will continue, but may experience instability or wrong behavior"
 )
 
 type ActivePodsFunc func() []*v1.Pod

pkg/kubelet/events/event.go

@@ -77,7 +77,6 @@ const (
 	FailedPrepareDynamicResources        = "FailedPrepareDynamicResources"
 	PossibleMemoryBackedVolumesOnDisk    = "PossibleMemoryBackedVolumesOnDisk"
 	CgroupV1                             = "CgroupV1"
-	CgroupV2                             = "CgroupV2"
 )
 
 // Image manager event reason list

pkg/kubelet/kubelet.go

@@ -52,7 +52,6 @@ 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"
@@ -121,7 +120,6 @@ 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"
@@ -1647,7 +1645,9 @@ func (kl *Kubelet) Run(updates <-chan kubetypes.PodUpdate) {
 		os.Exit(1)
 	}
 
-	kl.cgroupVersionCheck()
+	if err := kl.cgroupVersionCheck(); err != nil {
+		klog.V(2).InfoS("Warning: cgroup check", "error", err)
+	}
 
 	// Start volume manager
 	go kl.volumeManager.Run(kl.sourcesReady, wait.NeverStop)
@@ -3068,22 +3068,3 @@ func (kl *Kubelet) PrepareDynamicResources(pod *v1.Pod) error {
 func (kl *Kubelet) UnprepareDynamicResources(pod *v1.Pod) error {
 	return kl.containerManager.UnprepareDynamicResources(pod)
 }
-
-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))
-}

pkg/kubelet/kubelet_linux.go

@@ -0,0 +1,51 @@
+//go:build linux
+
+/*
+Copyright 2024 The Kubernetes Authors.
+
+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.
+*/
+
+package kubelet
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	v1 "k8s.io/api/core/v1"
+	"k8s.io/kubernetes/pkg/kubelet/cm"
+	"k8s.io/kubernetes/pkg/kubelet/cm/util"
+	"k8s.io/kubernetes/pkg/kubelet/events"
+	"k8s.io/kubernetes/pkg/kubelet/metrics"
+)
+
+func (kl *Kubelet) cgroupVersionCheck() error {
+	cgroupVersion := kl.containerManager.GetNodeConfig().CgroupVersion
+	metrics.CgroupVersion.Set(float64(cgroupVersion))
+	switch cgroupVersion {
+	case 1:
+		kl.recorder.Eventf(kl.nodeRef, v1.EventTypeWarning, events.CgroupV1, cm.CgroupV1MaintenanceModeWarning)
+		return errors.New(cm.CgroupV1MaintenanceModeWarning)
+	case 2:
+		cpustat := filepath.Join(util.CgroupRoot, "cpu.stat")
+		if _, err := os.Stat(cpustat); os.IsNotExist(err) {
+			// if `/sys/fs/cgroup/cpu.stat` does not exist, log a warning
+			return errors.New(cm.CgroupV2KernelWarning)
+		}
+	default:
+		return fmt.Errorf("unsupported cgroup version: %d", cgroupVersion)
+	}
+	return nil
+}

pkg/kubelet/kubelet_others.go

@@ -0,0 +1,25 @@
+//go:build !linux
+
+/*
+Copyright 2024 The Kubernetes Authors.
+
+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.
+*/
+
+package kubelet
+
+// cgroupVersionCheck performs a version check for the cgroup.
+// This method is not applicable for non-Linux operating systems.
+func (kl *Kubelet) cgroupVersionCheck() error {
+	return nil
+}

pkg/util/kernel/constants.go

@@ -54,8 +54,3 @@ 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"