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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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