[PATCH v3 4/4] tracing/user_events: Document multi-format flag

Beau Belgrave
2024-02-14T17:50:56Z

User programs can now ask user_events to handle the synchronization of
multiple different formats for an event with the same name via the new
USER_EVENT_REG_MULTI_FORMAT flag.

Add a section for USER_EVENT_REG_MULTI_FORMAT that explains the intended
purpose and caveats of using it. Explain how deletion works in these
cases and how to use /sys/kernel/tracing/dynamic_events for per-version
deletion.

Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
---
Documentation/trace/user_events.rst | 27 ++++++++++++++++++++++++++-
1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/Documentation/trace/user_events.rst b/Documentation/trace/user_events.rst
index d8f12442aaa6..1d5a7626e6a6 100644
--- a/Documentation/trace/user_events.rst
+++ b/Documentation/trace/user_events.rst
@@ -92,6 +92,24 @@ The following flags are currently supported.
process closes or unregisters the event. Requires CAP_PERFMON otherwise
-EPERM is returned.

++ USER_EVENT_REG_MULTI_FORMAT: The event can contain multiple formats. This
+ allows programs to prevent themselves from being blocked when their event
+ format changes and they wish to use the same name. When this flag is used the
+ tracepoint name will be in the new format of "name.unique_id" vs the older
+ format of "name". A tracepoint will be created for each unique pair of name
+ and format. This means if several processes use the same name and format,
+ they will use the same tracepoint. If yet another process uses the same name,
+ but a different format than the other processes, it will use a different
+ tracepoint with a new unique id. Recording programs need to scan tracefs for
+ the various different formats of the event name they are interested in
+ recording. The system name of the tracepoint will also use "user_events_multi"
+ instead of "user_events". This prevents single-format event names conflicting
+ with any multi-format event names within tracefs. The unique_id is output as
+ a hex string. Recording programs should ensure the tracepoint name starts with
+ the event name they registered and has a suffix that starts with . and only
+ has hex characters. For example to find all versions of the event "test" you
+ can use the regex "^test\.[0-9a-fA-F]+$".
+
Upon successful registration the following is set.

+ write_index: The index to use for this file descriptor that represents this
@@ -106,6 +124,9 @@ or perf record -e user_events:[name] when attaching/recording.
**NOTE:** The event subsystem name by default is "user_events". Callers should
not assume it will always be "user_events". Operators reserve the right in the
future to change the subsystem name per-process to accommodate event isolation.
+In addition if the USER_EVENT_REG_MULTI_FORMAT flag is used the tracepoint name
+will have a unique id appended to it and the system name will be
+"user_events_multi" as described above.

Command Format
^^^^^^^^^^^^^^
@@ -156,7 +177,11 @@ to request deletes than the one used for registration due to this.
to the event. If programs do not want auto-delete, they must use the
USER_EVENT_REG_PERSIST flag when registering the event. Once that flag is used
the event exists until DIAG_IOCSDEL is invoked. Both register and delete of an
-event that persists requires CAP_PERFMON, otherwise -EPERM is returned.
+event that persists requires CAP_PERFMON, otherwise -EPERM is returned. When
+there are multiple formats of the same event name, all events with the same
+name will be attempted to be deleted. If only a specific version is wanted to
+be deleted then the /sys/kernel/tracing/dynamic_events file should be used for
+that specific format of the event.

Unregistering
-------------
--
2.34.1