linux-kernel - [RFC 2/3] kernel/cpu: Mark nonboot cpus as inactive when shutting down nonboot cpus

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Message-ID: <20251022121345.23496-3-piliu@redhat.com>
Date: Wed, 22 Oct 2025 20:13:44 +0800
From: Pingfan Liu <piliu@...hat.com>
To: kexec@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Cc: Pingfan Liu <piliu@...hat.com>,
	Waiman Long <longman@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Pierre Gondois <pierre.gondois@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Baoquan He <bhe@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Valentin Schneider <vschneid@...hat.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Joel Granados <joel.granados@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: [RFC 2/3] kernel/cpu: Mark nonboot cpus as inactive when shutting down nonboot cpus

The previous patch lifted the deadline bandwidth check during the kexec
process, which raises a potential issue: as the number of online CPUs
decreases, DL tasks may be crowded onto a few CPUs, which may starve the
CPU hotplug kthread. As a result, the hot-removal cannot proceed in
practice.  On the other hand, as CPUs are offlined one by one, all tasks
will eventually be migrated to the kexec CPU.

Therefore, this patch marks all other CPUs as inactive to signal the
scheduler to migrate tasks to the kexec CPU during hot-removal.

Signed-off-by: Pingfan Liu <piliu@...hat.com>
Cc: Waiman Long <longman@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Pierre Gondois <pierre.gondois@....com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: kexec@...ts.infradead.org
To: linux-kernel@...r.kernel.org
---
 kernel/cpu.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index db9f6c539b28c..76aa0f784602b 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1546,6 +1546,16 @@ void smp_shutdown_nonboot_cpus(unsigned int primary_cpu)
 	if (!cpu_online(primary_cpu))
 		primary_cpu = cpumask_first(cpu_online_mask);
 
+	/*
+	 * Mark all other CPUs as inactive so the scheduler won't select them as
+	 * migration targets.
+	 */
+	for_each_online_cpu(cpu) {
+		if (cpu == primary_cpu)
+			continue;
+		set_cpu_active(cpu, false);
+	}
+
 	for_each_online_cpu(cpu) {
 		if (cpu == primary_cpu)
 			continue;
-- 
2.49.0