[PATCH] selftests/tracing: Have trigger-hist-poll.tc use sched_process_exit

[PATCH] selftests/tracing: Have trigger-hist-poll.tc use sched_process_exit

[Steven Rostedt]

From: Steven Rostedt <rostedt@goodmis.org>

Currently trigger-hist-poll.tc uses sched_process_free to test the polling
of the histogram file. The way it does that is to run sleep, then execute
the poll.c code that polls on the sched_process_free for up to 4 seconds
to test that when sleep triggers the sched_process_free trace event, it
will update the histogram and wake the poll.c code up.

The issue is that sched_process_free trace event is called by
delayed_put_task_struct() which is called after a RCU grace period has
ended. If CONFIG_RCU_LAZY is enabled, RCU callbacks are batched together
and do not execute right away. This causes the delayed_put_task_struct()
to be called after the poll.c function finishes and it will report an
error that it did not wake up on the event. That's because the event
didn't trigger during its wait time.

Use sched_process_exit instead, which is called when a process exits and
doesn't depend on RCU callbacks that may be delayed.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 .../selftests/ftrace/test.d/trigger/trigger-hist-poll.tc      | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/ftrace/test.d/trigger/trigger-hist-poll.tc b/tools/testing/selftests/ftrace/test.d/trigger/trigger-hist-poll.tc
index 8d275e3238d9..04eb8546fc07 100644
--- a/tools/testing/selftests/ftrace/test.d/trigger/trigger-hist-poll.tc
+++ b/tools/testing/selftests/ftrace/test.d/trigger/trigger-hist-poll.tc
@@ -1,7 +1,7 @@
 #!/bin/sh
 # SPDX-License-Identifier: GPL-2.0
 # description: event trigger - test poll wait on histogram
-# requires: set_event events/sched/sched_process_free/trigger events/sched/sched_process_free/hist
+# requires: set_event events/sched/sched_process_exit/trigger events/sched/sched_process_exit/hist
 # flags: instance
 
 POLL=${FTRACETEST_ROOT}/poll
@@ -11,7 +11,7 @@ if [ ! -x ${POLL} ]; then
   exit_unresolved
 fi
 
-EVENT=events/sched/sched_process_free/
+EVENT=events/sched/sched_process_exit/
 
 # Check poll ops is supported. Before implementing poll on hist file, it
 # returns soon with POLLIN | POLLOUT, but not POLLPRI.
-- 
2.53.0

Re: [PATCH] selftests/tracing: Have trigger-hist-poll.tc use sched_process_exit

[Steven Rostedt]

On Wed, 8 Jul 2026 16:34:36 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: Steven Rostedt <rostedt@goodmis.org>
> 
> Currently trigger-hist-poll.tc uses sched_process_free to test the polling
> of the histogram file. The way it does that is to run sleep, then execute
> the poll.c code that polls on the sched_process_free for up to 4 seconds
> to test that when sleep triggers the sched_process_free trace event, it
> will update the histogram and wake the poll.c code up.
> 
> The issue is that sched_process_free trace event is called by
> delayed_put_task_struct() which is called after a RCU grace period has
> ended. If CONFIG_RCU_LAZY is enabled, RCU callbacks are batched together
> and do not execute right away. This causes the delayed_put_task_struct()
> to be called after the poll.c function finishes and it will report an
> error that it did not wake up on the event. That's because the event
> didn't trigger during its wait time.
> 
> Use sched_process_exit instead, which is called when a process exits and
> doesn't depend on RCU callbacks that may be delayed.
> 
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> ---

I forgot to mention that I tested this with and without CONFIG_RCU_LAZY.
With that config disabled, the test passes. With it enabled, it always
fails.

-- Steve

Re: [PATCH] selftests/tracing: Have trigger-hist-poll.tc use sched_process_exit

[Steven Rostedt]

On Wed, 8 Jul 2026 16:35:39 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> I forgot to mention that I tested this with and without CONFIG_RCU_LAZY.
> With that config disabled, the test passes. With it enabled, it always
> fails.

And I also traced it with:

  trace-cmd set -p function_graph -l event_hist_poll -O funcgraph-retval

and it shows the sched_process_free being called for the sleep process
*after* the poll finishes.

-- Steve

Re: [PATCH] selftests/tracing: Have trigger-hist-poll.tc use sched_process_exit

[Masami Hiramatsu]

On Wed, 8 Jul 2026 16:37:31 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Wed, 8 Jul 2026 16:35:39 -0400
> Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > I forgot to mention that I tested this with and without CONFIG_RCU_LAZY.
> > With that config disabled, the test passes. With it enabled, it always
> > fails.
> 
> And I also traced it with:
> 
>   trace-cmd set -p function_graph -l event_hist_poll -O funcgraph-retval
> 
> and it shows the sched_process_free being called for the sleep process
> *after* the poll finishes.

Thanks for fix and tested!!

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

> 
> -- Steve


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>