kubeadm: set the kubelet cgroup driver to "systemd" during "init"

The kubeadm documentation instructs users to set the container
runtime driver to "systemd", since kubeadm manages a kubelet via
the systemd init system. The kubelet default however is "cgroupfs".
For new clusters set the driver to "systemd" unless the user
is explicit about it. The same defaulting would not happen
during "upgrade".

cmd/kubeadm/app/apis/kubeadm/types.go

@@ -451,6 +451,12 @@ type ComponentConfig interface {
 
 	// SetUserSupplied sets the state of the component config "user supplied" flag to, either true, or false.
 	SetUserSupplied(userSupplied bool)
+
+	// Set can be used to set the internal configuration in the ComponentConfig
+	Set(interface{})
+
+	// Get can be used to get the internal configuration in the ComponentConfig
+	Get() interface{}
 }
 
 // ComponentConfigMap is a map between a group name (as in GVK group) and a ComponentConfig

cmd/kubeadm/app/cmd/init.go

@@ -37,6 +37,7 @@ import (
 	phases "k8s.io/kubernetes/cmd/kubeadm/app/cmd/phases/init"
 	"k8s.io/kubernetes/cmd/kubeadm/app/cmd/phases/workflow"
 	cmdutil "k8s.io/kubernetes/cmd/kubeadm/app/cmd/util"
+	"k8s.io/kubernetes/cmd/kubeadm/app/componentconfigs"
 	kubeadmconstants "k8s.io/kubernetes/cmd/kubeadm/app/constants"
 	"k8s.io/kubernetes/cmd/kubeadm/app/features"
 	certsphase "k8s.io/kubernetes/cmd/kubeadm/app/phases/certs"
@@ -336,6 +337,12 @@ func newInitData(cmd *cobra.Command, args []string, options *initOptions, out io
 		return nil, err
 	}
 
+	// For new clusters we want to set the kubelet cgroup driver to "systemd" unless the user is explicit about it.
+	// Currently this cannot be as part of the kubelet defaulting (Default()) because the function is called for
+	// upgrades too, which can break existing nodes after a kubelet restart.
+	// TODO: https://github.com/kubernetes/kubeadm/issues/2376
+	componentconfigs.MutateCgroupDriver(&cfg.ClusterConfiguration)
+
 	ignorePreflightErrorsSet, err := validation.ValidateIgnorePreflightErrors(options.ignorePreflightErrors, cfg.NodeRegistration.IgnorePreflightErrors)
 	if err != nil {
 		return nil, err

cmd/kubeadm/app/cmd/upgrade/plan.go

@@ -91,6 +91,10 @@ func runPlan(flags *planFlags, args []string) error {
 		return errors.WithMessage(err, "[upgrade/versions] FATAL")
 	}
 
+	// Warn if the kubelet component config is missing an explicit 'cgroupDriver'.
+	// TODO: https://github.com/kubernetes/kubeadm/issues/2376
+	componentconfigs.WarnCgroupDriver(&cfg.ClusterConfiguration)
+
 	// No upgrades available
 	if len(availUpgrades) == 0 {
 		klog.V(1).Infoln("[upgrade/plan] Awesome, you're up-to-date! Enjoy!")

cmd/kubeadm/app/componentconfigs/fakeconfig_test.go

@@ -96,6 +96,14 @@ func (cc *clusterConfig) Unmarshal(docmap kubeadmapi.DocumentMap) error {
 	return cc.configBase.Unmarshal(docmap, &cc.config)
 }
 
+func (cc *clusterConfig) Get() interface{} {
+	return &cc.config
+}
+
+func (cc *clusterConfig) Set(cfg interface{}) {
+	cc.config = *cfg.(*kubeadmapiv1.ClusterConfiguration)
+}
+
 func (cc *clusterConfig) Default(_ *kubeadmapi.ClusterConfiguration, _ *kubeadmapi.APIEndpoint, _ *kubeadmapi.NodeRegistrationOptions) {
 	cc.config.ClusterName = "foo"
 	cc.config.KubernetesVersion = "bar"

cmd/kubeadm/app/componentconfigs/kubelet.go

@@ -17,8 +17,10 @@ limitations under the License.
 package componentconfigs
 
 import (
+	"fmt"
 	"path/filepath"
 
+	"github.com/pkg/errors"
 	"k8s.io/apimachinery/pkg/util/version"
 	clientset "k8s.io/client-go/kubernetes"
 	"k8s.io/klog/v2"
@@ -104,6 +106,14 @@ func (kc *kubeletConfig) Unmarshal(docmap kubeadmapi.DocumentMap) error {
 	return kc.configBase.Unmarshal(docmap, &kc.config)
 }
 
+func (kc *kubeletConfig) Get() interface{} {
+	return &kc.config
+}
+
+func (kc *kubeletConfig) Set(cfg interface{}) {
+	kc.config = *cfg.(*kubeletconfig.KubeletConfiguration)
+}
+
 func (kc *kubeletConfig) Default(cfg *kubeadmapi.ClusterConfiguration, _ *kubeadmapi.APIEndpoint, nodeRegOpts *kubeadmapi.NodeRegistrationOptions) {
 	const kind = "KubeletConfiguration"
 
@@ -231,3 +241,54 @@ func isServiceActive(name string) (bool, error) {
 	}
 	return initSystem.ServiceIsActive(name), nil
 }
+
+// TODO: https://github.com/kubernetes/kubeadm/issues/2376
+const cgroupDriverSystemd = "systemd"
+
+// MutateCgroupDriver can be called to set the KubeletConfiguration cgroup driver to systemd.
+// Currently this cannot be as part of Default() because the function is called for
+// upgrades too, which can break existing nodes after a kubelet restart.
+// TODO: https://github.com/kubernetes/kubeadm/issues/2376
+func MutateCgroupDriver(cfg *kubeadmapi.ClusterConfiguration) {
+	cc, k, err := getKubeletConfig(cfg)
+	if err != nil {
+		klog.Warningf(err.Error())
+		return
+	}
+	if len(k.CgroupDriver) == 0 {
+		klog.V(1).Infof("setting the KubeletConfiguration cgroupDriver to %q", cgroupDriverSystemd)
+		k.CgroupDriver = cgroupDriverSystemd
+		cc.Set(k)
+	}
+}
+
+// WarnCgroupDriver prints a warning in case the user is not explicit
+// about the cgroupDriver value in the KubeletConfiguration.
+// TODO: https://github.com/kubernetes/kubeadm/issues/2376
+func WarnCgroupDriver(cfg *kubeadmapi.ClusterConfiguration) {
+	_, k, err := getKubeletConfig(cfg)
+	if err != nil {
+		klog.Warningf(err.Error())
+		return
+	}
+	if len(k.CgroupDriver) == 0 {
+		klog.Warningf("The 'cgroupDriver' value in the KubeletConfiguration is empty. " +
+			"Starting from 1.22, 'kubeadm upgrade' will default an empty value to the 'systemd' cgroup driver. " +
+			"The cgroup driver between the container runtime and the kubelet must match! " +
+			"To learn more about this see: https://kubernetes.io/docs/setup/production-environment/container-runtimes/")
+	}
+}
+
+// TODO: https://github.com/kubernetes/kubeadm/issues/2376
+func getKubeletConfig(cfg *kubeadmapi.ClusterConfiguration) (kubeadmapi.ComponentConfig, *kubeletconfig.KubeletConfiguration, error) {
+	errStr := fmt.Sprintf("setting the KubeletConfiguration cgroupDriver to %q failed unexpectedly", cgroupDriverSystemd)
+	cc, ok := cfg.ComponentConfigs[KubeletGroup]
+	if !ok {
+		return nil, nil, errors.Errorf("%s: %s", errStr, "missing kubelet component config")
+	}
+	k, ok := cc.Get().(*kubeletconfig.KubeletConfiguration)
+	if !ok {
+		return nil, nil, errors.Errorf("%s: %s", errStr, "incompatible KubeletConfiguration")
+	}
+	return cc, k, nil
+}

cmd/kubeadm/app/componentconfigs/kubeproxy.go

@@ -83,6 +83,14 @@ func kubeProxyDefaultBindAddress(localAdvertiseAddress string) string {
 	return kubeadmapiv1beta2.DefaultProxyBindAddressv6
 }
 
+func (kp *kubeProxyConfig) Get() interface{} {
+	return &kp.config
+}
+
+func (kp *kubeProxyConfig) Set(cfg interface{}) {
+	kp.config = *cfg.(*kubeproxyconfig.KubeProxyConfiguration)
+}
+
 func (kp *kubeProxyConfig) Default(cfg *kubeadmapi.ClusterConfiguration, localAPIEndpoint *kubeadmapi.APIEndpoint, _ *kubeadmapi.NodeRegistrationOptions) {
 	const kind = "KubeProxyConfiguration"