[PATCH] sched/deadline: Fix stale throttling on de-/boosted tasks

[Lucas Stach]

When a boosted task gets throttled, what normally happens is that it's
immediately enqueued again with ENQUEUE_REPLENISH, which replenishes the
runtime and clears the dl_throttled flag. There is a special case however:
if the throttling happened on sched-out and the task has been deboosted in
the meantime, the replenish is skipped as the task will return to its
normal scheduling class. This leaves the task with the dl_throttled flag
set.

Now if the task gets boosted up to the deadline scheduling class again
while it is sleeping, it's still in the throttled state. The normal wakeup
however will enqueue the task with ENQUEUE_REPLENISH not set, so we don't
actually place it on the rq. Thus we end up with a task that is runnable,
but not actually on the rq and neither a immediate replenishment happens,
nor is the replenishment timer set up, so the task is stuck in
forever-throttled limbo.

Clear the dl_throttled flag before dropping back to the normal scheduling
class to fix this issue.

Signed-off-by: Lucas Stach <l.stach@xxxxxxxxxxxxxx>
---
This is the root cause and fix of the issue described at [1]. After working
on other stuff for the last few months, I finally was able to circle back
to this issue and gather the required data to pinpoint the failure mode.

[1] https://lkml.org/lkml/2020/3/20/765
---
 kernel/sched/deadline.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 3862a28cd05d..c19c1883d695 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1527,12 +1527,15 @@ static void enqueue_task_dl(struct rq *rq, struct task_struct *p, int flags)
 		pi_se = &pi_task->dl;
 	} else if (!dl_prio(p->normal_prio)) {
 		/*
-		 * Special case in which we have a !SCHED_DEADLINE task
-		 * that is going to be deboosted, but exceeds its
-		 * runtime while doing so. No point in replenishing
-		 * it, as it's going to return back to its original
-		 * scheduling class after this.
+		 * Special case in which we have a !SCHED_DEADLINE task that is going
+		 * to be deboosted, but exceeds its runtime while doing so. No point in
+		 * replenishing it, as it's going to return back to its original
+		 * scheduling class after this. If it has been throttled, we need to
+		 * clear the flag, otherwise the task may wake up as throttled after
+		 * being boosted again with no means to replenish the runtime and clear
+		 * the throttle.
 		 */
+		p->dl.dl_throttled = 0;
 		BUG_ON(!p->dl.dl_boosted || flags != ENQUEUE_REPLENISH);
 		return;
 	}
-- 
2.20.1