On 6/3/26 19:00, Steven Rostedt wrote:
> On Wed, 3 Jun 2026 18:26:24 +0200
> "David Hildenbrand (Arm)" <[email protected]> wrote:
>
>> Yeah, I was fearing that when I read in [2]:
>>
>> "It has become clear in the past that this promise extends to
>> tracepoints, most notably in 2011 when a tracepoint change broke
>> powertop and had to be reverted."
>
> Technically the issue is with trace events and not tracepoints. The
> difference is that a trace event is created via the TRACE_EVENT() macro
> which defines what is to be collected from the tracepoint and exposes that
> information to tracefs which applications can easily see.
>
> A tracepoint is simply the hook in the code that you can attach to. Trace
> events create a callback from that hook to extract the data from the
> tracepoint to fill in the fields.
>
>>
>> Which means that I now also fully understand
>>
>> "Some kernel maintainers prohibit or severely restrict the addition of
>> tracepoints to their subsystems out of fear that a similar thing could
>> happen to them. "
>>
>> Whatever the result of this discussion will be, I'll try to document it.
>
> You can still create a tracepoint without creating a trace event by using
> the DECLARE_TRACE() macro. The scheduler subsystem uses that quite
> extensively. That creates a tracepoint without exposing it to tracefs. The
> runtime verifier uses these hooks to monitor the scheduler.
>
> But you can still connect to these tracepoints from tracefs via a tprobe. A
> tprobe hooks to tracepoints that you need the source code to find (just
> like a fprobe hooks to any function). Thus applications *can't* rely on
> them because there's nothing there to tell you it exists or not.
Thanks, that makes sense!
So, would it be fair to say that, in general, what's exposed through
/sys/kernel/tracing/events/
is stable ABI?
Would the following be sufficient to avoid a full revert and the dependency on
CONFIG_RAS?
diff --git a/include/trace/events/memory-failure.h
b/include/trace/events/memory-failure.h
index aa57cc8f896b..c46b17602578 100644
--- a/include/trace/events/memory-failure.h
+++ b/include/trace/events/memory-failure.h
@@ -1,6 +1,7 @@
/* SPDX-License-Identifier: GPL-2.0 */
#undef TRACE_SYSTEM
-#define TRACE_SYSTEM memory_failure
+/* Some user space relies on ras/memory_failure_event */
+#define TRACE_SYSTEM ras
#define TRACE_INCLUDE_FILE memory-failure
#if !defined(_TRACE_MEMORY_FAILURE_H) || defined(TRACE_HEADER_MULTI_READ)
--
Cheers,
David
