Troubleshooting operating system issues - Troubleshooting | Support

Investigating kernel crashes
Debugging Ignition failures

OpenShift Container Platform runs on RHCOS. You can follow these procedures to troubleshoot problems related to the operating system.

Investigating kernel crashes

The kdump service, included in kexec-tools, provides a crash-dumping mechanism. You can use this service to save the contents of the system’s memory for later analysis.

The kdump service is a Technology Preview feature only. Technology Preview features are not supported with Red Hat production service level agreements (SLAs) and might not be functionally complete. Red Hat does not recommend using them in production. These features provide early access to upcoming product features, enabling customers to test functionality and provide feedback during the development process.

For more information about the support scope of Red Hat Technology Preview features, see https://access.redhat.com/support/offerings/techpreview/.

Enabling kdump

RHCOS ships with kexec-tools, but manual configuration is required to enable kdump.

Procedure

Perform the following steps to enable kdump on RHCOS:

To reserve memory for the crash kernel during the first kernel booting, provide kernel arguments by entering the following command:
```
# rpm-ostree kargs --append='crashkernel=256M'
```
Optional: To write the crash dump over the network or to some other location, rather than to the default local /var/crash location, edit the /etc/kdump.conf configuration file.

Network dumps are required when using LUKS. kdump does not support local crash dumps on LUKS-encrypted devices.

For details on configuring the kdump service, see the comments in /etc/sysconfig/kdump, /etc/kdump.conf, and the kdump.conf manual page. Also refer to the RHEL kdump documentation for further information on configuring the dump target.
Enable the kdump systemd service:
```
# systemctl enable kdump.service
```
Reboot your system:
```
# systemctl reboot
```
Ensure that kdump has loaded a crash kernel by checking that the kdump.service has started and exited successfully and that cat /sys/kernel/kexec_crash_loaded prints 1.

Enabling kdump on day-1

The kdump service is intended to be enabled per node to debug kernel problems. Because there are costs to having kdump enabled, and these costs accumulate with each additional kdump-enabled node, it is recommended that kdump only be enabled on each node as needed. Potential costs of enabling kdump on each node include:

Less available RAM due to memory being reserved for the crash kernel.
Node unavailability while the kernel is dumping the core.
Additional storage space being used to store the crash dumps.
Not being production-ready because the kdump service is in Technology Preview.

If you are aware of the downsides and trade-offs of having the kdump service enabled, it is possible to enable kdump in a cluster-wide fashion. Although machine-specific machine configs are not yet supported, you can perform the previous steps through a systemd unit in a MachineConfig object on day-1 and have kdump enabled on all nodes in the cluster. You can create a MachineConfig object and inject that object into the set of manifest files used by Ignition during cluster setup. See "Customizing nodes" in the Installing → Installation configuration section for more information and examples on how to use Ignition configs.

Procedure

Create a MachineConfig object for cluster-wide configuration:

Create a Butane config file, 99-worker-kdump.bu, that configures and enables kdump:

variant: openshift
version: 4.9.0
metadata:
  name: 99-worker-kdump (1)
  labels:
    machineconfiguration.openshift.io/role: worker (1)
openshift:
  kernel_arguments: (2)
    - crashkernel=256M
storage:
  files:
    - path: /etc/kdump.conf (3)
      mode: 0644
      overwrite: true
      contents:
        inline: |
          path /var/crash
          core_collector makedumpfile -l --message-level 7 -d 31

    - path: /etc/sysconfig/kdump (4)
      mode: 0644
      overwrite: true
      contents:
        inline: |
          KDUMP_COMMANDLINE_REMOVE="hugepages hugepagesz slub_debug quiet log_buf_len swiotlb"
          KDUMP_COMMANDLINE_APPEND="irqpoll nr_cpus=1 reset_devices cgroup_disable=memory mce=off numa=off udev.children-max=2 panic=10 rootflags=nofail acpi_no_memhotplug transparent_hugepage=never nokaslr novmcoredd hest_disable"
          KEXEC_ARGS="-s"
          KDUMP_IMG="vmlinuz"

systemd:
  units:
    - name: kdump.service
      enabled: true

1	Replace `worker` with `master` in both locations when creating a `MachineConfig` object for control plane nodes.
2	Provide kernel arguments to reserve memory for the crash kernel. You can add other kernel arguments if necessary.
3	If you want to change the contents of `/etc/kdump.conf` from the default, include this section and modify the `inline` subsection accordingly.
4	If you want to change the contents of `/etc/sysconfig/kdump` from the default, include this section and modify the `inline` subsection accordingly.

Use Butane to generate a machine config YAML file, 99-worker-kdump.yaml, containing the configuration to be delivered to the nodes:
```
$ butane 99-worker-kdump.bu -o 99-worker-kdump.yaml
```
Put the YAML file into manifests during cluster setup. You can also create this MachineConfig object after cluster setup with the YAML file:
```
$ oc create -f ./99-worker-kdump.yaml
```

Testing the kdump configuration

See the Testing the kdump configuration section in the RHEL documentation for kdump.

Analyzing a core dump

See the Analyzing a core dump section in the RHEL documentation for kdump.

It is recommended to perform vmcore analysis on a separate RHEL system.

Additional resources

Setting up kdump in RHEL
Linux kernel documentation for kdump
kdump.conf(5) — a manual page for the /etc/kdump.conf configuration file containing the full documentation of available options
kexec(8) — a manual page for the kexec package
Red Hat Knowledgebase article regarding kexec and kdump

Debugging Ignition failures

If a machine cannot be provisioned, Ignition fails and RHCOS will boot into the emergency shell. Use the following procedure to get debugging information.

Procedure

Run the following command to show which service units failed:
```
$ systemctl --failed
```
Optional: Run the following command on an individual service unit to find out more information:
```
$ journalctl -u <unit>.service
```