tracing: Cache ":mod:" events for modules not loaded yet

When the :mod: command is written into /sys/kernel/tracing/set_event (or
that file within an instance), if the module specified after the ":mod:"
is not yet loaded, it will store that string internally. When the module
is loaded, it will enable the events as if the module was loaded when the
string was written into the set_event file.

This can also be useful to enable events that are in the init section of
the module, as the events are enabled before the init section is executed.

This also works on the kernel command line:

 trace_event=:mod:<module>

Will enable the events for <module> when it is loaded.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: https://lore.kernel.org/20250116143533.514730995@goodmis.org
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

1 files changed, 3 insertions, 1 deletions

diff --git a/Documentation/trace/events.rst b/Documentation/trace/events.rst
index 3db57516eb86..2d88a2acacc0 100644
--- a/Documentation/trace/events.rst
+++ b/Documentation/trace/events.rst
@@ -60,7 +60,9 @@ a specific module::
 
 	# echo ':mod:<module>' > /sys/kernel/tracing/set_event
 
-Will enable all events in the module ``<module>``.
+Will enable all events in the module ``<module>``.  If the module is not yet
+loaded, the string will be saved and when a module is that matches ``<module>``
+is loaded, then it will apply the enabling of events then.
 
 The text before ``:mod:`` will be parsed to specify specific events that the
 module creates::