lib/percpu_counter: Convert to hotplug state machine

Install the callbacks via the state machine and let the core invoke
the callbacks on the already online CPUs.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: rt@linutronix.de
Link: http://lkml.kernel.org/r/20161103145021.28528-5-bigeasy@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/cpuhotplug.h

@@ -33,6 +33,7 @@ enum cpuhp_state {
 	CPUHP_FS_BUFF_DEAD,
 	CPUHP_PRINTK_DEAD,
 	CPUHP_MM_MEMCQ_DEAD,
+	CPUHP_PERCPU_CNT_DEAD,
 	CPUHP_WORKQUEUE_PREP,
 	CPUHP_POWER_NUMA_PREPARE,
 	CPUHP_HRTIMERS_PREPARE,

lib/percpu_counter.c

@@ -158,25 +158,21 @@ EXPORT_SYMBOL(percpu_counter_destroy);
 int percpu_counter_batch __read_mostly = 32;
 EXPORT_SYMBOL(percpu_counter_batch);
 
-static void compute_batch_value(void)
+static int compute_batch_value(unsigned int cpu)
 {
 	int nr = num_online_cpus();
 
 	percpu_counter_batch = max(32, nr*2);
+	return 0;
 }
 
-static int percpu_counter_hotcpu_callback(struct notifier_block *nb,
-					unsigned long action, void *hcpu)
+static int percpu_counter_cpu_dead(unsigned int cpu)
 {
 #ifdef CONFIG_HOTPLUG_CPU
-	unsigned int cpu;
 	struct percpu_counter *fbc;
 
-	compute_batch_value();
-	if (action != CPU_DEAD && action != CPU_DEAD_FROZEN)
-		return NOTIFY_OK;
+	compute_batch_value(cpu);
 
-	cpu = (unsigned long)hcpu;
 	spin_lock_irq(&percpu_counters_lock);
 	list_for_each_entry(fbc, &percpu_counters, list) {
 		s32 *pcount;
@@ -190,7 +186,7 @@ static int percpu_counter_hotcpu_callback(struct notifier_block *nb,
 	}
 	spin_unlock_irq(&percpu_counters_lock);
 #endif
-	return NOTIFY_OK;
+	return 0;
 }
 
 /*
@@ -222,8 +218,15 @@ EXPORT_SYMBOL(__percpu_counter_compare);
 
 static int __init percpu_counter_startup(void)
 {
-	compute_batch_value();
-	hotcpu_notifier(percpu_counter_hotcpu_callback, 0);
+	int ret;
+
+	ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "lib/percpu_cnt:online",
+				compute_batch_value, NULL);
+	WARN_ON(ret < 0);
+	ret = cpuhp_setup_state_nocalls(CPUHP_PERCPU_CNT_DEAD,
+					"lib/percpu_cnt:dead", NULL,
+					percpu_counter_cpu_dead);
+	WARN_ON(ret < 0);
 	return 0;
 }
 module_init(percpu_counter_startup);