Patch "sched_ext: Don't kick CPUs running higher classes" has been added to the 6.18-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    sched_ext: Don't kick CPUs running higher classes

to the 6.18-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     sched_ext-don-t-kick-cpus-running-higher-classes.patch
and it can be found in the queue-6.18 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


From stable+bounces-212753-greg=kroah.com@vger.kernel.org Thu Jan 29 10:26:03 2026
From: Christian Loehle <christian.loehle@arm.com>
Date: Thu, 29 Jan 2026 09:25:45 +0000
Subject: sched_ext: Don't kick CPUs running higher classes
To: stable@vger.kernel.org, tj@kernel.org
Cc: arighi@nvidia.com, void@manifault.com, sched-ext@lists.linux.dev, Christian Loehle <christian.loehle@arm.com>
Message-ID: <20260129092546.49128-2-christian.loehle@arm.com>

From: Tejun Heo <tj@kernel.org>

commit a9c1fbbd6dadbaa38c157a07d5d11005460b86b9 upstream.

When a sched_ext scheduler tries to kick a CPU, the CPU may be running a
higher class task. sched_ext has no control over such CPUs. A sched_ext
scheduler couldn't have expected to get access to the CPU after kicking it
anyway. Skip kicking when the target CPU is running a higher class.

Reviewed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Christian Loehle <christian.loehle@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 kernel/sched/ext.c |   11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -5164,18 +5164,23 @@ static bool kick_one_cpu(s32 cpu, struct
 {
 	struct rq *rq = cpu_rq(cpu);
 	struct scx_rq *this_scx = &this_rq->scx;
+	const struct sched_class *cur_class;
 	bool should_wait = false;
 	unsigned long flags;
 
 	raw_spin_rq_lock_irqsave(rq, flags);
+	cur_class = rq->curr->sched_class;
 
 	/*
 	 * During CPU hotplug, a CPU may depend on kicking itself to make
-	 * forward progress. Allow kicking self regardless of online state.
+	 * forward progress. Allow kicking self regardless of online state. If
+	 * @cpu is running a higher class task, we have no control over @cpu.
+	 * Skip kicking.
 	 */
-	if (cpu_online(cpu) || cpu == cpu_of(this_rq)) {
+	if ((cpu_online(cpu) || cpu == cpu_of(this_rq)) &&
+	    !sched_class_above(cur_class, &ext_sched_class)) {
 		if (cpumask_test_cpu(cpu, this_scx->cpus_to_preempt)) {
-			if (rq->curr->sched_class == &ext_sched_class)
+			if (cur_class == &ext_sched_class)
 				rq->curr->scx.slice = 0;
 			cpumask_clear_cpu(cpu, this_scx->cpus_to_preempt);
 		}


Patches currently in stable-queue which might be from christian.loehle@arm.com are

queue-6.18/sched_ext-don-t-kick-cpus-running-higher-classes.patch
queue-6.18/sched_ext-fix-scx_kick_wait-to-work-reliably.patch