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, 25 insertions, 0 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index cb55d4247e65..a0b11201a7b4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6654,6 +6654,23 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
 }
 #endif /* SCHED_PROXY_EXEC */
 
+static inline void proxy_tag_curr(struct rq *rq, struct task_struct *owner)
+{
+	if (!sched_proxy_exec())
+		return;
+	/*
+	 * pick_next_task() calls set_next_task() on the chosen task
+	 * at some point, which ensures it is not push/pullable.
+	 * However, the chosen/donor task *and* the mutex owner form an
+	 * atomic pair wrt push/pull.
+	 *
+	 * Make sure owner we run is not pushable. Unfortunately we can
+	 * only deal with that by means of a dequeue/enqueue cycle. :-/
+	 */
+	dequeue_task(rq, owner, DEQUEUE_NOCLOCK | DEQUEUE_SAVE);
+	enqueue_task(rq, owner, ENQUEUE_NOCLOCK | ENQUEUE_RESTORE);
+}
+
 /*
  * __schedule() is the main scheduler function.
  *
@@ -6798,6 +6815,10 @@ picked:
 		 * changes to task_struct made by pick_next_task().
 		 */
 		RCU_INIT_POINTER(rq->curr, next);
+
+		if (!task_current_donor(rq, next))
+			proxy_tag_curr(rq, next);
+
 		/*
 		 * The membarrier system call requires each architecture
 		 * to have a full memory barrier after updating
@@ -6832,6 +6853,10 @@ picked:
 		/* Also unlocks the rq: */
 		rq = context_switch(rq, prev, next, &rf);
 	} else {
+		/* In case next was already curr but just got blocked_donor */
+		if (!task_current_donor(rq, next))
+			proxy_tag_curr(rq, next);
+
 		rq_unpin_lock(rq, &rf);
 		__balance_callbacks(rq);
 		raw_spin_rq_unlock_irq(rq);