I've been tracking down an issue on a ~5.17ish kernel where:

  CPUx                           CPUy

  <DL task p0 owns an rtmutex M>
  <p0 depletes its runtime, gets throttled>
  <rq switches to the idle task>
				 <DL task p1 blocks on M, boost/replenish p0>
				 <No call to resched_curr() happens here>

  [idle task keeps running here until *something*
   accidentally sets TIF_NEED_RESCHED]

On that kernel, it is quite easy to trigger using rt-tests's deadline_test
[1] with the test running on isolated CPUs (this reduces the chance of
something unrelated setting TIF_NEED_RESCHED on the idle tasks, making the
issue even more obvious as the hung task detector chimes in).

I haven't been able to reproduce this using a mainline kernel, even if I
revert

  2972e3050e35 ("tracing: Make trace_marker{,_raw} stream-like")

which gets rid of the lock involved in the above test, *but* I cannot
convince myself the issue isn't there from looking at the code.

Make prio_changed_dl() issue a reschedule if the current task isn't a
deadline one. While at it, ensure a reschedule is emitted when a
queued-but-not-current task gets boosted with an earlier deadline that
current's.

[1]: https://git.kernel.org/pub/scm/utils/rt-tests/rt-tests.git
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
---
 kernel/sched/deadline.c | 45 ++++++++++++++++++++++++++---------------
 1 file changed, 29 insertions(+), 16 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 0d97d54276cc8..faa382ea084c1 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2663,17 +2663,28 @@ static void switched_to_dl(struct rq *rq, struct task_struct *p)
 static void prio_changed_dl(struct rq *rq, struct task_struct *p,
 			    int oldprio)
 {
-	if (task_on_rq_queued(p) || task_current(rq, p)) {
-#ifdef CONFIG_SMP
-		/*
-		 * This might be too much, but unfortunately
-		 * we don't have the old deadline value, and
-		 * we can't argue if the task is increasing
-		 * or lowering its prio, so...
-		 */
-		if (!rq->dl.overloaded)
-			deadline_queue_pull_task(rq);
+	if (!task_on_rq_queued(p))
+		return;
+
+	/*
+	 * We don't know if p has a earlier or later deadline, so let's blindly
+	 * set a (maybe not needed) rescheduling point.
+	 */
+	if (!IS_ENABLED(CONFIG_SMP)) {
+		resched_curr(rq);
+		return;
+	}
 
+	/*
+	 * This might be too much, but unfortunately
+	 * we don't have the old deadline value, and
+	 * we can't argue if the task is increasing
+	 * or lowering its prio, so...
+	 */
+	if (!rq->dl.overloaded)
+		deadline_queue_pull_task(rq);
+
+	if (task_current(rq, p)) {
 		/*
 		 * If we now have a earlier deadline task than p,
 		 * then reschedule, provided p is still on this
@@ -2681,14 +2692,16 @@ static void prio_changed_dl(struct rq *rq, struct task_struct *p,
 		 */
 		if (dl_time_before(rq->dl.earliest_dl.curr, p->dl.deadline))
 			resched_curr(rq);
-#else
+	} else {
 		/*
-		 * Again, we don't know if p has a earlier
-		 * or later deadline, so let's blindly set a
-		 * (maybe not needed) rescheduling point.
+		 * Current may not be deadline in case p was throttled but we
+		 * have just replenished it (e.g. rt_mutex_setprio()).
+		 *
+		 * Otherwise, if p was given an earlier deadline, reschedule.
 		 */
-		resched_curr(rq);
-#endif /* CONFIG_SMP */
+		if (!dl_task(rq->curr) ||
+		    dl_time_before(p->dl.deadline, rq->curr->dl.deadline))
+			resched_curr(rq);
 	}
 }
 
-- 
2.31.1

Hi,

On 02/02/23 18:28, Valentin Schneider wrote:
> I've been tracking down an issue on a ~5.17ish kernel where:
> 
>   CPUx                           CPUy
> 
>   <DL task p0 owns an rtmutex M>
>   <p0 depletes its runtime, gets throttled>
>   <rq switches to the idle task>
> 				 <DL task p1 blocks on M, boost/replenish p0>
> 				 <No call to resched_curr() happens here>
> 
>   [idle task keeps running here until *something*
>    accidentally sets TIF_NEED_RESCHED]
> 
> On that kernel, it is quite easy to trigger using rt-tests's deadline_test
> [1] with the test running on isolated CPUs (this reduces the chance of
> something unrelated setting TIF_NEED_RESCHED on the idle tasks, making the
> issue even more obvious as the hung task detector chimes in).
> 
> I haven't been able to reproduce this using a mainline kernel, even if I
> revert
> 
>   2972e3050e35 ("tracing: Make trace_marker{,_raw} stream-like")
> 
> which gets rid of the lock involved in the above test, *but* I cannot
> convince myself the issue isn't there from looking at the code.
> 
> Make prio_changed_dl() issue a reschedule if the current task isn't a
> deadline one. While at it, ensure a reschedule is emitted when a
> queued-but-not-current task gets boosted with an earlier deadline that
> current's.

As discussed offline I agree this needs fixing, but .. :)

> [1]: https://git.kernel.org/pub/scm/utils/rt-tests/rt-tests.git
> Signed-off-by: Valentin Schneider <vschneid@redhat.com>
> ---
>  kernel/sched/deadline.c | 45 ++++++++++++++++++++++++++---------------
>  1 file changed, 29 insertions(+), 16 deletions(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 0d97d54276cc8..faa382ea084c1 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -2663,17 +2663,28 @@ static void switched_to_dl(struct rq *rq, struct task_struct *p)
>  static void prio_changed_dl(struct rq *rq, struct task_struct *p,
>  			    int oldprio)
>  {
> -	if (task_on_rq_queued(p) || task_current(rq, p)) {
> -#ifdef CONFIG_SMP

Doesn't this break UP? Don't think earlierst_dl etc are defined in UP.

Thanks,
Juri

On 03/02/23 08:06, Juri Lelli wrote:
> Hi,
>
> On 02/02/23 18:28, Valentin Schneider wrote:
>> I've been tracking down an issue on a ~5.17ish kernel where:
>>
>>   CPUx                           CPUy
>>
>>   <DL task p0 owns an rtmutex M>
>>   <p0 depletes its runtime, gets throttled>
>>   <rq switches to the idle task>
>>                               <DL task p1 blocks on M, boost/replenish p0>
>>                               <No call to resched_curr() happens here>
>>
>>   [idle task keeps running here until *something*
>>    accidentally sets TIF_NEED_RESCHED]
>>
>> On that kernel, it is quite easy to trigger using rt-tests's deadline_test
>> [1] with the test running on isolated CPUs (this reduces the chance of
>> something unrelated setting TIF_NEED_RESCHED on the idle tasks, making the
>> issue even more obvious as the hung task detector chimes in).
>>
>> I haven't been able to reproduce this using a mainline kernel, even if I
>> revert
>>
>>   2972e3050e35 ("tracing: Make trace_marker{,_raw} stream-like")
>>
>> which gets rid of the lock involved in the above test, *but* I cannot
>> convince myself the issue isn't there from looking at the code.
>>
>> Make prio_changed_dl() issue a reschedule if the current task isn't a
>> deadline one. While at it, ensure a reschedule is emitted when a
>> queued-but-not-current task gets boosted with an earlier deadline that
>> current's.
>
> As discussed offline I agree this needs fixing, but .. :)
>
>> [1]: https://git.kernel.org/pub/scm/utils/rt-tests/rt-tests.git
>> Signed-off-by: Valentin Schneider <vschneid@redhat.com>
>> ---
>>  kernel/sched/deadline.c | 45 ++++++++++++++++++++++++++---------------
>>  1 file changed, 29 insertions(+), 16 deletions(-)
>>
>> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
>> index 0d97d54276cc8..faa382ea084c1 100644
>> --- a/kernel/sched/deadline.c
>> +++ b/kernel/sched/deadline.c
>> @@ -2663,17 +2663,28 @@ static void switched_to_dl(struct rq *rq, struct task_struct *p)
>>  static void prio_changed_dl(struct rq *rq, struct task_struct *p,
>>                          int oldprio)
>>  {
>> -	if (task_on_rq_queued(p) || task_current(rq, p)) {
>> -#ifdef CONFIG_SMP
>
> Doesn't this break UP? Don't think earlierst_dl etc are defined in UP.
>

Indeed, I thought myself clever by getting rid of the ifdefs...

> Thanks,
> Juri