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.. SPDX-License-Identifier: GPL-2.0
Debugging Kernel Shutdown Hangs with pstore
+++++++++++++++++++++++++++++++++++++++++++
Overview
========
If the system hangs while shutting down, the kernel logs may need to be
retrieved to debug the issue.
On systems that have a UART available, it is best to configure the kernel to use
this UART for kernel console output.
If a UART isn't available, the ``pstore`` subsystem provides a mechanism to
persist this data across a system reset, allowing it to be retrieved on the next
boot.
Kernel Configuration
====================
To enable ``pstore`` and enable saving kernel ring buffer logs, set the
following kernel configuration options:
* ``CONFIG_PSTORE=y``
* ``CONFIG_PSTORE_CONSOLE=y``
Additionally, enable a backend to store the data. Depending upon your platform
some potential options include:
* ``CONFIG_EFI_VARS_PSTORE=y``
* ``CONFIG_PSTORE_RAM=y``
* ``CONFIG_CHROMEOS_PSTORE=y``
* ``CONFIG_PSTORE_BLK=y``
Kernel Command-line Parameters
==============================
Add these parameters to your kernel command line:
* ``printk.always_kmsg_dump=Y``
* Forces the kernel to dump the entire message buffer to pstore during
shutdown
* ``efi_pstore.pstore_disable=N``
* For EFI-based systems, ensures the EFI backend is active
Userspace Interaction and Log Retrieval
=======================================
On the next boot after a hang, pstore logs will be available in the pstore
filesystem (``/sys/fs/pstore``) and can be retrieved by userspace.
On systemd systems, the ``systemd-pstore`` service will help do the following:
#. Locate pstore data in ``/sys/fs/pstore``
#. Read and save it to ``/var/lib/systemd/pstore``
#. Clear pstore data for the next event
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