linux-kernel - [PATCH v3 2/3] sched_ext: Fix cpu_released while RT task and SCX task are scheduled concurrently

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Message-ID: <ea7ad66c0e6140979ce941189f5a93cc@honor.com>
Date: Sun, 20 Jul 2025 09:38:50 +0000
From: liuwenfang <liuwenfang@...or.com>
To: 'Tejun Heo' <tj@...nel.org>
CC: 'David Vernet' <void@...ifault.com>, 'Andrea Righi' <arighi@...dia.com>,
	'Changwoo Min' <changwoo@...lia.com>, 'Ingo Molnar' <mingo@...hat.com>,
	'Peter Zijlstra' <peterz@...radead.org>, 'Juri Lelli'
	<juri.lelli@...hat.com>, 'Vincent Guittot' <vincent.guittot@...aro.org>,
	'Dietmar Eggemann' <dietmar.eggemann@....com>, 'Steven Rostedt'
	<rostedt@...dmis.org>, 'Ben Segall' <bsegall@...gle.com>, 'Mel Gorman'
	<mgorman@...e.de>, 'Valentin Schneider' <vschneid@...hat.com>,
	"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>
Subject: [PATCH v3 2/3] sched_ext: Fix cpu_released while RT task and SCX task
 are scheduled concurrently

Supposed RT task(RT1) is running on CPU0 and RT task(RT2) is awakened on CPU1,
RT1 becomes sleep and SCX task(SCX1) will be dispatched to CPU0, RT2 will be
placed on CPU0:

CPU0(schedule)                                     CPU1(try_to_wake_up)
set_current_state(TASK_INTERRUPTIBLE)              try_to_wake_up # RT2
__schedule                                           select_task_rq # CPU0 is selected
LOCK rq(0)->lock # lock CPU0 rq                        ttwu_queue
  deactivate_task # RT1                                  LOCK rq(0)->lock # busy waiting
    pick_next_task # no more RT tasks on rq                 |
      prev_balance                                          |
        balance_scx                                         |
          balance_one                                       |
            rq->scx.cpu_released = false;                   |
              consume_global_dsq                            |
                consume_dispatch_q                          |
                  consume_remote_task                       |
                    UNLOCK rq(0)->lock                      V
                                                         LOCK rq(0)->lock # succ
                    deactivate_task # SCX1               ttwu_do_activate
                    LOCK rq(0)->lock # busy waiting      activate_task # RT2 equeued
                       |                                 UNLOCK rq(0)->lock
                       V
                    LOCK rq(0)->lock # succ
                    activate_task # SCX1
      pick_task # RT2 is picked
      put_prev_set_next_task # prev is RT1, next is RT2, rq->scx.cpu_released = false;
UNLOCK rq(0)->lock

At last, RT2 will be running on CPU0 with rq->scx.cpu_released being false!

So, Add the scx_next_task_picked () and check sched class again to fix the value
of rq->scx.cpu_released.

Signed-off-by: Wenfang Liu liuwenfang@...or.com
---
 kernel/sched/ext.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 93e03b7d0..ddf4bd060 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -3236,6 +3236,11 @@ void scx_put_prev_set_next(struct rq *rq, struct task_struct *prev,
 	 */
 	smp_store_release(&rq->scx.pnt_seq, rq->scx.pnt_seq + 1);
 #endif
+
+	if (next->sched_class == &ext_sched_class)
+		return;
+
+	switch_class(rq, next);
 }
 
 static void put_prev_task_scx(struct rq *rq, struct task_struct *p,
@@ -3258,7 +3263,7 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p,
 		 */
 		if (p->scx.slice && !scx_rq_bypassing(rq)) {
 			dispatch_enqueue(&rq->scx.local_dsq, p, SCX_ENQ_HEAD);
-			goto switch_class;
+			return;
 		}
 
 		/*
@@ -3274,10 +3279,6 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p,
 			do_enqueue_task(rq, p, 0, -1);
 		}
 	}
-
-switch_class:
-	if (next && next->sched_class != &ext_sched_class)
-		switch_class(rq, next);
 }
 
 static struct task_struct *first_local_task(struct rq *rq)
-- 
2.17.1