Add documentation explaining the msgid feature in netconsole.
This feature appends unique id to the userdata dictionary. The message ID is populated from a per-target 32 bit counter which is incremented for each message sent to the target. This allows a target to detect if messages are dropped before reaching the target.
Signed-off-by: Gustavo Luiz Duarte gustavold@gmail.com --- Documentation/networking/netconsole.rst | 32 ++++++++++++++++++++++++++++++++ 1 file changed, 32 insertions(+)
diff --git a/Documentation/networking/netconsole.rst b/Documentation/networking/netconsole.rst index a0076b542e9c..59cb9982afe6 100644 --- a/Documentation/networking/netconsole.rst +++ b/Documentation/networking/netconsole.rst @@ -340,6 +340,38 @@ In this example, the message was sent by CPU 42. cpu=42 # kernel-populated value
+Message ID auto population in userdata +-------------------------------------- + +Within the netconsole configfs hierarchy, there is a file named `msgid_enabled` +located in the `userdata` directory. This file controls the message ID +auto-population feature, which assigns a numeric id to each message sent to a +given target and appends the ID to userdata dictionary in every message sent. + +The message ID is generated using a per-target 32 bit counter that is +incremented for every message sent to the target. Note that this counter will +eventually wrap around after reaching uint32_t max value, so the message ID is +not globally unique over time. However, it can still be used by the target to +detect if messages were dropped before reaching the target by identifying gaps +in the sequence of IDs. + +It is important to distinguish message IDs from the message <sequnum> field. +Some kernel messages may never reach netconsole (for example, due to printk +rate limiting). Thus, a gap in <sequnum> cannot be solely relied upon to +indicate that a message was dropped during transmission, as it may never have +been sent via netconsole. The message ID, on the other hand, is only assigned +to messages that are actually transmitted via netconsole. + +Example:: + + echo "This is message #1" > /dev/kmsg + echo "This is message #2" > /dev/kmsg + 13,434,54928466,-;This is message #1 + msgid=1 + 13,435,54934019,-;This is message #2 + msgid=2 + + Extended console: =================