Commit 3ca55ca2 authored Apr 02, 2025 by Andrii Nakryiko Committed by Andrew Morton May 11, 2025

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>

parent 82f2b0b9

include/trace/events/sched.h

+30 −4

Original line number	Diff line number	Diff line
		@@ -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:

kernel/exit.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -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);