linux-kernel - [tip: sched/core] sched: Fix faulty assertion in sched_change

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Message-ID: <176596899373.510.17191516261088315233.tip-bot2@tip-bot2>
Date: Wed, 17 Dec 2025 10:56:33 -0000
From: "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Naresh Kamboju <naresh.kamboju@...aro.org>,
 Linux Kernel Functional Testing <lkft@...aro.org>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: sched/core] sched: Fix faulty assertion in sched_change_end()

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     1862d8e264def8425d682f1177e22f9fe7d947ea
Gitweb:        https://git.kernel.org/tip/1862d8e264def8425d682f1177e22f9fe7d947ea
Author:        Peter Zijlstra <peterz@...radead.org>
AuthorDate:    Wed, 17 Dec 2025 11:24:11 +01:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Wed, 17 Dec 2025 11:41:18 +01:00

sched: Fix faulty assertion in sched_change_end()

Commit 47efe2ddccb1f ("sched/core: Add assertions to QUEUE_CLASS") added an
assert to sched_change_end() verifying that a class demotion would result in a
reschedule.

As it turns out; rt_mutex_setprio() does not force a resched on class
demontion. Furthermore, this is only relevant to running tasks.

Change the warning into a reschedule and make sure to only do so for running
tasks.

Fixes: 47efe2ddccb1f ("sched/core: Add assertions to QUEUE_CLASS")
Reported-by: Naresh Kamboju <naresh.kamboju@...aro.org>
Tested-by:  Linux Kernel Functional Testing <lkft@...aro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://patch.msgid.link/20251216141725.GW3707837@noisy.programming.kicks-ass.net
---
 kernel/sched/core.c | 33 +++++++++++++++++----------------
 1 file changed, 17 insertions(+), 16 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7d0a862..5b17d8e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10833,23 +10833,24 @@ void sched_change_end(struct sched_change_ctx *ctx)
 		if (p->sched_class->switched_to)
 			p->sched_class->switched_to(rq, p);
 
-		/*
-		 * If this was a class promotion; let the old class know it
-		 * got preempted. Note that none of the switch*_from() methods
-		 * know the new class and none of the switch*_to() methods
-		 * know the old class.
-		 */
-		if (ctx->running && sched_class_above(p->sched_class, ctx->class)) {
-			rq->next_class->wakeup_preempt(rq, p, 0);
-			rq->next_class = p->sched_class;
+		if (ctx->running) {
+			/*
+			 * If this was a class promotion; let the old class
+			 * know it got preempted. Note that none of the
+			 * switch*_from() methods know the new class and none
+			 * of the switch*_to() methods know the old class.
+			 */
+			if (sched_class_above(p->sched_class, ctx->class)) {
+				rq->next_class->wakeup_preempt(rq, p, 0);
+				rq->next_class = p->sched_class;
+			}
+			/*
+			 * If this was a degradation in class; make sure to
+			 * reschedule.
+			 */
+			if (sched_class_above(ctx->class, p->sched_class))
+				resched_curr(rq);
 		}
-
-		/*
-		 * If this was a degradation in class someone should have set
-		 * need_resched by now.
-		 */
-		WARN_ON_ONCE(sched_class_above(ctx->class, p->sched_class) &&
-			     !test_tsk_need_resched(p));
 	} else {
 		p->sched_class->prio_changed(rq, p, ctx->prio);
 	}