[tip: sched/core] sched/tracing: Report TASK_RTLOCK_WAIT tasks as TASK_UNINTERRUPTIBLE

[tip-bot2 for Valentin Schneider]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     25795ef6299f07ce3838f3253a9cb34f64efcfae
Gitweb:        https://git.kernel.org/tip/25795ef6299f07ce3838f3253a9cb34f64efcfae
Author:        Valentin Schneider <valentin.schneider@xxxxxxx>
AuthorDate:    Thu, 20 Jan 2022 16:25:20 
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 01 Mar 2022 16:18:39 +01:00

sched/tracing: Report TASK_RTLOCK_WAIT tasks as TASK_UNINTERRUPTIBLE

TASK_RTLOCK_WAIT currently isn't part of TASK_REPORT, thus a task blocking
on an rtlock will appear as having a task state == 0, IOW TASK_RUNNING.

The actual state is saved in p->saved_state, but reading it after reading
p->__state has a few issues:
o that could still be TASK_RUNNING in the case of e.g. rt_spin_lock
o ttwu_state_match() might have changed that to TASK_RUNNING

As pointed out by Eric, adding TASK_RTLOCK_WAIT to TASK_REPORT implies
exposing a new state to userspace tools which way not know what to do with
them. The only information that needs to be conveyed here is that a task is
waiting on an rt_mutex, which matches TASK_UNINTERRUPTIBLE - there's no
need for a new state.

Reported-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
Signed-off-by: Valentin Schneider <valentin.schneider@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Link: https://lore.kernel.org/r/20220120162520.570782-3-valentin.schneider@xxxxxxx
---
 include/linux/sched.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 457c8a0..78b606c 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1630,6 +1630,14 @@ static inline unsigned int __task_state_index(unsigned int tsk_state,
 	if (tsk_state == TASK_IDLE)
 		state = TASK_REPORT_IDLE;
 
+	/*
+	 * We're lying here, but rather than expose a completely new task state
+	 * to userspace, we can make this appear as if the task has gone through
+	 * a regular rt_mutex_lock() call.
+	 */
+	if (tsk_state == TASK_RTLOCK_WAIT)
+		state = TASK_UNINTERRUPTIBLE;
+
 	return fls(state);
 }