Commit be39617e authored by Valentin Schneider's avatar Valentin Schneider Committed by Peter Zijlstra
Browse files

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: default avatarValentin Schneider <valentin.schneider@arm.com>
Signed-off-by: default avatarConnor O'Brien <connoro@google.com>
Signed-off-by: default avatarJohn Stultz <jstultz@google.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: default avatarK Prateek Nayak <kprateek.nayak@amd.com>
Link: https://lkml.kernel.org/r/20250712033407.2383110-8-jstultz@google.com
parent be41bde4

kernel/sched/core.c

+25 −0
Original line number Diff line number Diff line
@@ -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 @@ static void __sched notrace __schedule(int sched_mode) 
		 * 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 @@ static void __sched notrace __schedule(int sched_mode) 
		/* 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);

kernel/sched/deadline.c

+7 −0
Original line number Diff line number Diff line
@@ -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);

}

kernel/sched/rt.c

+5 −0
Original line number Diff line number Diff line
@@ -1440,6 +1440,9 @@ enqueue_task_rt(struct rq *rq, struct task_struct *p, int flags) 


	enqueue_rt_entity(rt_se, flags);



	if (task_is_blocked(p))

		return;



	if (!task_current(rq, p) && p->nr_cpus_allowed > 1)

		enqueue_pushable_task(rq, p);

}
@@ -1716,6 +1719,8 @@ static void put_prev_task_rt(struct rq *rq, struct task_struct *p, struct task_s 


	update_rt_rq_load_avg(rq_clock_pelt(rq), rq, 1);



	if (task_is_blocked(p))

		return;

	/*

	 * The previous task needs to be made eligible for pushing

	 * if it is still active