sched: Fix proxy/current (push,pull)ability

Proxy execution forms atomic pairs of tasks: The waiting donor
task (scheduling context) and a proxy (execution context). The
donor task, along with the rest of the blocked chain, follows
the proxy wrt CPU placement.

They can be the same task, in which case push/pull doesn't need any
modification. When they are different, however,
FIFO1 & FIFO42:

	      ,->  RT42
	      |     | blocked-on
	      |     v
blocked_donor |   mutex
	      |     | owner
	      |     v
	      `--  RT1

   RT1
   RT42

  CPU0            CPU1
   ^                ^
   |                |
  overloaded    !overloaded
  rq prio = 42  rq prio = 0

RT1 is eligible to be pushed to CPU1, but should that happen it will
"carry" RT42 along. Clearly here neither RT1 nor RT42 must be seen as
push/pullable.

Unfortunately, only the donor task is usually dequeued from the rq,
and the proxy'ed execution context (rq->curr) remains on the rq.
This can cause RT1 to be selected for migration from logic like the
rt pushable_list.

Thus, adda a dequeue/enqueue cycle on the proxy task before __schedule
returns, which allows the sched class logic to avoid adding the now
current task to the pushable_list.

Furthermore, tasks becoming blocked on a mutex don't need an explicit
dequeue/enqueue cycle to be made (push/pull)able: they have to be running
to block on a mutex, thus they will eventually hit put_prev_task().

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Connor O'Brien <connoro@google.com>
Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Link: https://lkml.kernel.org/r/20250712033407.2383110-8-jstultz@google.com

1 files changed, 7 insertions, 0 deletions

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 1af06e48227d..e2d51f4306b3 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2121,6 +2121,9 @@ static void enqueue_task_dl(struct rq *rq, struct task_struct *p, int flags)
 	if (dl_server(&p->dl))
 		return;
 
+	if (task_is_blocked(p))
+		return;
+
 	if (!task_current(rq, p) && !p->dl.dl_throttled && p->nr_cpus_allowed > 1)
 		enqueue_pushable_dl_task(rq, p);
 }
@@ -2415,6 +2418,10 @@ static void put_prev_task_dl(struct rq *rq, struct task_struct *p, struct task_s
 	update_curr_dl(rq);
 
 	update_dl_rq_load_avg(rq_clock_pelt(rq), rq, 1);
+
+	if (task_is_blocked(p))
+		return;
+
 	if (on_dl_rq(&p->dl) && p->nr_cpus_allowed > 1)
 		enqueue_pushable_dl_task(rq, p);
 }