Configuring a cgroup driver

This page explains how to configure the kubelet cgroup driver to match the container runtime cgroup driver for kubeadm clusters.

Before you begin

You should be familiar with the Kubernetes container runtime requirements.

Configuring the container runtime cgroup driver

The Container runtimes page explains that the systemd driver is recommended for kubeadm based setups instead of the cgroupfs driver, because kubeadm manages the kubelet as a systemd service.

The page also provides details on how to setup a number of different container runtimes with the systemd driver by default.

Configuring the kubelet cgroup driver

kubeadm allows you to pass a KubeletConfiguration structure during kubeadm init. This KubeletConfiguration can include the cgroupDriver field which controls the cgroup driver of the kubelet.

Note:

FEATURE STATE: Kubernetes v1.21 [stable]

If the user is not setting the cgroupDriver field under KubeletConfiguration, kubeadm init will default it to systemd.

A minimal example of configuring the field explicitly:

# kubeadm-config.yaml
kind: ClusterConfiguration
apiVersion: kubeadm.k8s.io/v1beta2
kubernetesVersion: v1.21.0
---
kind: KubeletConfiguration
apiVersion: kubelet.config.k8s.io/v1beta1
cgroupDriver: systemd

Such a configuration file can then be passed to the kubeadm command:

kubeadm init --config kubeadm-config.yaml

Note:

Kubeadm uses the same KubeletConfiguration for all nodes in the cluster. The KubeletConfiguration is stored in a ConfigMap object under the kube-system namespace.

Executing the sub commands init, join and upgrade would result in kubeadm writing the KubeletConfiguration as a file under /var/lib/kubelet/config.yaml and passing it to the local node kubelet.

Using the cgroupfs driver

As this guide explains using the cgroupfs driver with kubeadm is not recommended.

To continue using cgroupfs and to prevent kubeadm upgrade from modifying the KubeletConfiguration cgroup driver on existing setups, you must be explicit about its value. This applies to a case where you do not wish future versions of kubeadm to apply the systemd driver by default.

See the below section on "Modify the kubelet ConfigMap" for details on how to be explicit about the value.

If you wish to configure a container runtime to use the cgroupfs driver, you must refer to the documentation of the container runtime of your choice.

Migrating to the systemd driver

To change the cgroup driver of an existing kubeadm cluster to systemd in-place, a similar procedure to a kubelet upgrade is required. This must include both steps outlined below.

Note: Alternatively, it is possible to replace the old nodes in the cluster with new ones that use the systemd driver. This requires executing only the first step below before joining the new nodes and ensuring the workloads can safely move to the new nodes before deleting the old nodes.

Modify the kubelet ConfigMap

• Find the kubelet ConfigMap name using kubectl get cm -n kube-system | grep kubelet-config.

• Call kubectl edit cm kubelet-config-x.yy -n kube-system (replace x.yy with the Kubernetes version).

• Either modify the existing cgroupDriver value or add a new field that looks like this:

  cgroupDriver: systemd
  

  This field must be present under the kubelet: section of the ConfigMap.

Update the cgroup driver on all nodes

For each node in the cluster:

• Drain the node using kubectl drain <node-name> --ignore-daemonsets
• Stop the kubelet using systemctl stop kubelet
• Stop the container runtime
• Modify the container runtime cgroup driver to systemd
• Set cgroupDriver: systemd in /var/lib/kubelet/config.yaml
• Start the container runtime
• Start the kubelet using systemctl start kubelet
• Uncordon the node using kubectl uncordon <node-name>

Execute these steps on nodes one at a time to ensure workloads have sufficient time to schedule on different nodes.

Once the process is complete ensure that all nodes and workloads are healthy.