sched: Allow hotplug notifiers to be setup early

Prevent the SMP scheduler related notifiers to be executed before the smp
scheduler is initialized and install them early.

This is a preparatory change for further consolidation of the hotplug notifier
maze.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: rt@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/sched/core.c

@@ -5195,6 +5195,8 @@ int task_can_attach(struct task_struct *p,
 
 #ifdef CONFIG_SMP
 
+static bool sched_smp_initialized __read_mostly;
+
 #ifdef CONFIG_NUMA_BALANCING
 /* Migrate current task p to target_cpu */
 int migrate_task_to(struct task_struct *p, int target_cpu)
@@ -5513,25 +5515,6 @@ int sched_cpu_starting(unsigned int cpu)
 	return 0;
 }
 
-static int __init migration_init(void)
-{
-	void *cpu = (void *)(long)smp_processor_id();
-	int err;
-
-	/* Initialize migration for the boot CPU */
-	err = migration_call(&migration_notifier, CPU_UP_PREPARE, cpu);
-	BUG_ON(err == NOTIFY_BAD);
-	migration_call(&migration_notifier, CPU_ONLINE, cpu);
-	register_cpu_notifier(&migration_notifier);
-
-	/* Register cpu active notifiers */
-	cpu_notifier(sched_cpu_active, CPU_PRI_SCHED_ACTIVE);
-	cpu_notifier(sched_cpu_inactive, CPU_PRI_SCHED_INACTIVE);
-
-	return 0;
-}
-early_initcall(migration_init);
-
 static cpumask_var_t sched_domains_tmpmask; /* sched_domains_mutex */
 
 #ifdef CONFIG_SCHED_DEBUG
@@ -6711,6 +6694,9 @@ static int sched_domains_numa_masks_update(struct notifier_block *nfb,
 {
 	int cpu = (long)hcpu;
 
+	if (!sched_smp_initialized)
+		return NOTIFY_DONE;
+
 	switch (action & ~CPU_TASKS_FROZEN) {
 	case CPU_ONLINE:
 		sched_domains_numa_masks_set(cpu);
@@ -7129,6 +7115,9 @@ static int num_cpus_frozen;	/* used to mark begin/end of suspend/resume */
 static int cpuset_cpu_active(struct notifier_block *nfb, unsigned long action,
 			     void *hcpu)
 {
+	if (!sched_smp_initialized)
+		return NOTIFY_DONE;
+
 	switch (action) {
 	case CPU_ONLINE_FROZEN:
 	case CPU_DOWN_FAILED_FROZEN:
@@ -7169,6 +7158,9 @@ static int cpuset_cpu_inactive(struct notifier_block *nfb, unsigned long action,
 	bool overflow;
 	int cpus;
 
+	if (!sched_smp_initialized)
+		return NOTIFY_DONE;
+
 	switch (action) {
 	case CPU_DOWN_PREPARE:
 		rcu_read_lock_sched();
@@ -7216,10 +7208,6 @@ void __init sched_init_smp(void)
 		cpumask_set_cpu(smp_processor_id(), non_isolated_cpus);
 	mutex_unlock(&sched_domains_mutex);
 
-	hotcpu_notifier(sched_domains_numa_masks_update, CPU_PRI_SCHED_ACTIVE);
-	hotcpu_notifier(cpuset_cpu_active, CPU_PRI_CPUSET_ACTIVE);
-	hotcpu_notifier(cpuset_cpu_inactive, CPU_PRI_CPUSET_INACTIVE);
-
 	init_hrtick();
 
 	/* Move init over to a non-isolated CPU */
@@ -7230,7 +7218,32 @@ void __init sched_init_smp(void)
 
 	init_sched_rt_class();
 	init_sched_dl_class();
+	sched_smp_initialized = true;
 }
+
+static int __init migration_init(void)
+{
+	void *cpu = (void *)(long)smp_processor_id();
+	int err;
+
+	/* Initialize migration for the boot CPU */
+	err = migration_call(&migration_notifier, CPU_UP_PREPARE, cpu);
+	BUG_ON(err == NOTIFY_BAD);
+	migration_call(&migration_notifier, CPU_ONLINE, cpu);
+	register_cpu_notifier(&migration_notifier);
+
+	/* Register cpu active notifiers */
+	cpu_notifier(sched_cpu_active, CPU_PRI_SCHED_ACTIVE);
+	cpu_notifier(sched_cpu_inactive, CPU_PRI_SCHED_INACTIVE);
+
+	hotcpu_notifier(sched_domains_numa_masks_update, CPU_PRI_SCHED_ACTIVE);
+	hotcpu_notifier(cpuset_cpu_active, CPU_PRI_CPUSET_ACTIVE);
+	hotcpu_notifier(cpuset_cpu_inactive, CPU_PRI_CPUSET_INACTIVE);
+
+	return 0;
+}
+early_initcall(migration_init);
+
 #else
 void __init sched_init_smp(void)
 {