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netconsole: docs: Add documentation for CPU number auto-population

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Update the netconsole documentation to explain the new feature that
allows automatic population of the CPU number.

The key changes include introducing a new section titled "CPU number
auto population in userdata", explaining how to enable the CPU number
auto-population feature by writing to the "populate_cpu_nr" file in the
netconsole configfs hierarchy.

This documentation update ensures users are aware of the new CPU number
auto-population functionality and how to leverage it for better
demultiplexing and visibility of parallel netconsole output.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Breno Leitao 2025-02-06 03:05:59 -08:00 committed by David S. Miller
parent 12fd83ca44
commit a7aec70a90
1 changed files with 45 additions and 0 deletions
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Documentation/networking/netconsole.rst
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@ -17,6 +17,8 @@ Release prepend support by Breno Leitao <leitao@debian.org>, Jul 7 2023
Userdata append support by Matthew Wood <thepacketgeek@gmail.com>, Jan 22 2024
Sysdata append support by Breno Leitao <leitao@debian.org>, Jan 15 2025
Please send bug reports to Matt Mackall <mpm@selenic.com>
Satyam Sharma <satyam.sharma@gmail.com>, and Cong Wang <xiyou.wangcong@gmail.com>
@ -238,6 +240,49 @@ Delete `userdata` entries with `rmdir`::
It is recommended to not write user data values with newlines.
CPU number auto population in userdata
--------------------------------------
Inside the netconsole configfs hierarchy, there is a file called
`cpu_nr` under the `userdata` directory. This file is used to enable or disable
the automatic CPU number population feature. This feature automatically
populates the CPU number that is sending the message.
To enable the CPU number auto-population::
echo 1 > /sys/kernel/config/netconsole/target1/userdata/cpu_nr
When this option is enabled, the netconsole messages will include an additional
line in the userdata field with the format `cpu=<cpu_number>`. This allows the
receiver of the netconsole messages to easily differentiate and demultiplex
messages originating from different CPUs, which is particularly useful when
dealing with parallel log output.
Example::
echo "This is a message" > /dev/kmsg
12,607,22085407756,-;This is a message
cpu=42
In this example, the message was sent by CPU 42.
.. note::
If the user has set a conflicting `cpu` key in the userdata dictionary,
both keys will be reported, with the kernel-populated entry appearing after
the user one. For example::
# User-defined CPU entry
mkdir -p /sys/kernel/config/netconsole/target1/userdata/cpu
echo "1" > /sys/kernel/config/netconsole/target1/userdata/cpu/value
Output might look like::
12,607,22085407756,-;This is a message
cpu=1
cpu=42 # kernel-populated value
Extended console:
=================