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

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 <[email protected]>
Reviewed-by: Simon Horman <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: leitao

Documentation/networking/netconsole.rst

@@ -17,6 +17,8 @@ Release prepend support by Breno Leitao <[email protected]>, Jul 7 2023
 
 Userdata append support by Matthew Wood <[email protected]>, Jan 22 2024
 
+Sysdata append support by Breno Leitao <[email protected]>, Jan 15 2025
+
 Please send bug reports to Matt Mackall <[email protected]>
 Satyam Sharma <[email protected]>, and Cong Wang <[email protected]>
 
@@ -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:
 =================