[PATCH v7 12/23] sched: Fix proxy/current (push,pull)ability

[John Stultz]

From: Valentin Schneider <valentin.schneider@xxxxxxx>

Proxy execution forms atomic pairs of tasks: The selected task
(scheduling context) and a proxy (execution context). The
selected 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 selected 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.

This patch adds 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().

XXX: pinned tasks becoming unblocked should be removed from the push/pull
lists, but those don't get to see __schedule() straight away.

Cc: Joel Fernandes <joelaf@xxxxxxxxxx>
Cc: Qais Yousef <qyousef@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Ben Segall <bsegall@xxxxxxxxxx>
Cc: Zimuzo Ezeozue <zezeozue@xxxxxxxxxx>
Cc: Youssef Esmat <youssefesmat@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Cc: Metin Kaya <Metin.Kaya@xxxxxxx>
Cc: Xuewen Yan <xuewen.yan94@xxxxxxxxx>
Cc: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: kernel-team@xxxxxxxxxxx
Signed-off-by: Valentin Schneider <valentin.schneider@xxxxxxx>
Signed-off-by: Connor O'Brien <connoro@xxxxxxxxxx>
Signed-off-by: John Stultz <jstultz@xxxxxxxxxx>
---
v3:
* Tweaked comments & commit message
v5:
* Minor simplifications to utilize the fix earlier
  in the patch series.
* Rework the wording of the commit message to match selected/
  proxy terminology and expand a bit to make it more clear how
  it works.
v6:
* Droped now-unused proxied value, to be re-added later in the
  series when it is used, as caught by Dietmar
v7:
* Unused function argument fixup
* Commit message nit pointed out by Metin Kaya
* Droped unproven unlikely() and use sched_proxy_exec()
  in proxy_tag_curr, suggested by Metin Kaya
---
 kernel/sched/core.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 12f5a0618328..f6bf3b62194c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6674,6 +6674,23 @@ find_proxy_task(struct rq *rq, struct task_struct *next, struct rq_flags *rf)
 }
 #endif /* SCHED_PROXY_EXEC */
 
+static inline void proxy_tag_curr(struct rq *rq, struct task_struct *next)
+{
+	if (sched_proxy_exec()) {
+		/*
+		 * pick_next_task() calls set_next_task() on the selected task
+		 * at some point, which ensures it is not push/pullable.
+		 * However, the selected task *and* the ,mutex owner form an
+		 * atomic pair wrt push/pull.
+		 *
+		 * Make sure owner is not pushable. Unfortunately we can only
+		 * deal with that by means of a dequeue/enqueue cycle. :-/
+		 */
+		dequeue_task(rq, next, DEQUEUE_NOCLOCK | DEQUEUE_SAVE);
+		enqueue_task(rq, next, ENQUEUE_NOCLOCK | ENQUEUE_RESTORE);
+	}
+}
+
 /*
  * __schedule() is the main scheduler function.
  *
@@ -6796,6 +6813,10 @@ static void __sched notrace __schedule(unsigned int sched_mode)
 		 * changes to task_struct made by pick_next_task().
 		 */
 		RCU_INIT_POINTER(rq->curr, next);
+
+		if (!task_current_selected(rq, next))
+			proxy_tag_curr(rq, next);
+
 		/*
 		 * The membarrier system call requires each architecture
 		 * to have a full memory barrier after updating
@@ -6820,6 +6841,10 @@ static void __sched notrace __schedule(unsigned 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_selected(rq, next))
+			proxy_tag_curr(rq, next);
+
 		rq_unpin_lock(rq, &rf);
 		__balance_callbacks(rq);
 		raw_spin_rq_unlock_irq(rq);
-- 
2.43.0.472.g3155946c3a-goog