linux-kernel - sched_ext: Don't kick CPUs running higher classes

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Message-ID: <20251021210354.89570-2-tj@kernel.org>
Date: Tue, 21 Oct 2025 11:03:53 -1000
From: Tejun Heo <tj@...nel.org>
To: David Vernet <void@...ifault.com>,
	Andrea Righi <arighi@...dia.com>,
	Changwoo Min <changwoo@...lia.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	sched-ext@...ts.linux.dev,
	bpf@...r.kernel.org,
	Tejun Heo <tj@...nel.org>
Subject: sched_ext: Don't kick CPUs running higher classes

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.

Signed-off-by: Tejun Heo <tj@...nel.org>
---
 kernel/sched/ext.c |   11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -5122,18 +5122,23 @@ 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);
 		}