[PATCH v3] sched_ext: Replace rq_lock() to raw_spin_rq_lock() in scx_ops_bypass()

[Changwoo Min]

scx_ops_bypass() iterates all CPUs to re-enqueue all the scx tasks.
For each CPU, it acquires a lock using rq_lock() regardless of whether
a CPU is offline or the CPU is currently running a task in a higher
scheduler class (e.g., deadline). The rq_lock() is supposed to be used
for online CPUs, and the use of rq_lock() may trigger an unnecessary
warning in rq_pin_lock(). Therefore, replace rq_lock() to
raw_spin_rq_lock() in scx_ops_bypass().

Without this change, we observe the following warning:

===== START =====
[    6.615205] rq->balance_callback && rq->balance_callback != &balance_push_callback
[    6.615208] WARNING: CPU: 2 PID: 0 at kernel/sched/sched.h:1730 __schedule+0x1130/0x1c90
=====  END  =====

Fixes: 0e7ffff1b811 ("scx: Fix raciness in scx_ops_bypass()")
Signed-off-by: Changwoo Min <changwoo@xxxxxxxxxx>
---

ChangeLog v2 -> v3:
  - Trim long warning messages for readability.
  - Properly add the Fixes tag.

ChangeLog v1 -> v2:
  - Add warning messages without this change.
  - Use resched_curr() instead of resched_cpu() to avoid redundant locking (Andrea)

 kernel/sched/ext.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 8fe64c27004e..cb6eb49d16be 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -4803,10 +4803,9 @@ static void scx_ops_bypass(bool bypass)
 	 */
 	for_each_possible_cpu(cpu) {
 		struct rq *rq = cpu_rq(cpu);
-		struct rq_flags rf;
 		struct task_struct *p, *n;
 
-		rq_lock(rq, &rf);
+		raw_spin_rq_lock(rq);
 
 		if (bypass) {
 			WARN_ON_ONCE(rq->scx.flags & SCX_RQ_BYPASSING);
@@ -4822,7 +4821,7 @@ static void scx_ops_bypass(bool bypass)
 		 * sees scx_rq_bypassing() before moving tasks to SCX.
 		 */
 		if (!scx_enabled()) {
-			rq_unlock(rq, &rf);
+			raw_spin_rq_unlock(rq);
 			continue;
 		}
 
@@ -4842,10 +4841,11 @@ static void scx_ops_bypass(bool bypass)
 			sched_enq_and_set_task(&ctx);
 		}
 
-		rq_unlock(rq, &rf);
-
 		/* resched to restore ticks and idle state */
-		resched_cpu(cpu);
+		if (cpu_online(cpu) || cpu == smp_processor_id())
+			resched_curr(rq);
+
+		raw_spin_rq_unlock(rq);
 	}
 
 	atomic_dec(&scx_ops_breather_depth);
-- 
2.47.1