exit: move and extend sched_process_exit() tracepoint

It is useful to be able to access current->mm at task exit to, say, record
a bunch of VMA information right before the task exits (e.g., for stack
symbolization reasons when dealing with short-lived processes that exit in
the middle of profiling session).  Currently, trace_sched_process_exit()
is triggered after exit_mm() which resets current->mm to NULL making this
tracepoint unsuitable for inspecting and recording task's
mm_struct-related data when tracing process lifetimes.

There is a particularly suitable place, though, right after
taskstats_exit() is called, but before we do exit_mm() and other exit_*()
resource teardowns.  taskstats performs a similar kind of accounting that
some applications do with BPF, and so co-locating them seems like a good
fit.  So that's where trace_sched_process_exit() is moved with this patch.

Also, existing trace_sched_process_exit() tracepoint is notoriously
missing `group_dead` flag that is certainly useful in practice and some of
our production applications have to work around this.  So plumb
`group_dead` through while at it, to have a richer and more complete
tracepoint.

Note that we can't use sched_process_template anymore, and so we use
TRACE_EVENT()-based tracepoint definition.  But all the field names and
order, as well as assign and output logic remain intact.  We just add one
extra field at the end in backwards-compatible way.

[andrii@kernel.org: document sched_process_exit and sched_process_template relation]
  Link: https://lkml.kernel.org/r/20250403174120.4087794-1-andrii@kernel.org
Link: https://lkml.kernel.org/r/20250402180925.90914-1-andrii@kernel.org
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Suggested-by: Ingo Molnar <mingo@kernel.org>
Cc: Alexander Potapenko <glider@google.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 31 insertions, 5 deletions

diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
index 8994e97d86c1..3bec9fb73a36 100644
--- a/include/trace/events/sched.h
+++ b/include/trace/events/sched.h
@@ -326,11 +326,37 @@ DEFINE_EVENT(sched_process_template, sched_process_free,
 	     TP_ARGS(p));
 
 /*
- * Tracepoint for a task exiting:
+ * Tracepoint for a task exiting.
+ * Note, it's a superset of sched_process_template and should be kept
+ * compatible as much as possible. sched_process_exits has an extra
+ * `group_dead` argument, so sched_process_template can't be used,
+ * unfortunately, just like sched_migrate_task above.
  */
-DEFINE_EVENT(sched_process_template, sched_process_exit,
-	     TP_PROTO(struct task_struct *p),
-	     TP_ARGS(p));
+TRACE_EVENT(sched_process_exit,
+
+	TP_PROTO(struct task_struct *p, bool group_dead),
+
+	TP_ARGS(p, group_dead),
+
+	TP_STRUCT__entry(
+		__array(	char,	comm,	TASK_COMM_LEN	)
+		__field(	pid_t,	pid			)
+		__field(	int,	prio			)
+		__field(	bool,	group_dead		)
+	),
+
+	TP_fast_assign(
+		memcpy(__entry->comm, p->comm, TASK_COMM_LEN);
+		__entry->pid		= p->pid;
+		__entry->prio		= p->prio; /* XXX SCHED_DEADLINE */
+		__entry->group_dead	= group_dead;
+	),
+
+	TP_printk("comm=%s pid=%d prio=%d group_dead=%s",
+		  __entry->comm, __entry->pid, __entry->prio,
+		  __entry->group_dead ? "true" : "false"
+	)
+);
 
 /*
  * Tracepoint for waiting on task to unschedule:
diff --git a/kernel/exit.c b/kernel/exit.c
index 1b51dc099f1e..f1db86dcbeb1 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -936,12 +936,12 @@ void __noreturn do_exit(long code)
 
 	tsk->exit_code = code;
 	taskstats_exit(tsk, group_dead);
+	trace_sched_process_exit(tsk, group_dead);
 
 	exit_mm();
 
 	if (group_dead)
 		acct_process();
-	trace_sched_process_exit(tsk);
 
 	exit_sem(tsk);
 	exit_shm(tsk);