From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Fri, 29 Nov 2019 17:24:55 +0100
Subject: [PATCH] sched/core: migrate_enable() must access takedown_cpu_task on
 !HOTPLUG_CPU
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git
Git-commit: da77ceac3d20f27310a07a7c346a4ee6b40d6c28
Patch-mainline: Queued in subsystem maintainer repository
References: SLE Realtime Extension

The variable takedown_cpu_task is never declared/used on !HOTPLUG_CPU
except for migrate_enable(). This leads to a link error.

Don't use takedown_cpu_task in !HOTPLUG_CPU.

Reported-by: Dick Hollenbeck <dick@softplc.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Daniel Wagner <dwagner@suse.de>
---
 kernel/cpu.c        |    2 ++
 kernel/sched/core.c |    2 ++
 2 files changed, 4 insertions(+)

--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -887,7 +887,9 @@ static int take_cpu_down(void *_param)
 	return 0;
 }
 
+#ifdef CONFIG_PREEMPT_RT
 struct task_struct *takedown_cpu_task;
+#endif
 
 static int takedown_cpu(unsigned int cpu)
 {
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8129,9 +8129,11 @@ void migrate_enable(void)
 
 	p->migrate_disable = 0;
 	rq->nr_pinned--;
+#ifdef CONFIG_HOTPLUG_CPU
 	if (rq->nr_pinned == 0 && unlikely(!cpu_active(cpu)) &&
 	    takedown_cpu_task)
 		wake_up_process(takedown_cpu_task);
+#endif
 
 	if (!p->migrate_disable_scheduled)
 		goto out;