[patch 18/20] cpu/hotplug: Let upcoming cpu bring itself fully up

[Thomas Gleixner]

Let the upcoming cpu kick the hotplug thread and let itself complete the
bringup. That way the controll side can just wait for the completion or later
when we made the hotplug machinery async not care at all.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 include/linux/cpuhotplug.h |    9 ++++--
 kernel/cpu.c               |   66 +++++++++++++++++++++++++--------------------
 kernel/sched/idle.c        |    2 +
 3 files changed, 45 insertions(+), 32 deletions(-)

Index: b/include/linux/cpuhotplug.h
===================================================================
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -10,9 +10,6 @@ enum cpuhp_state {
 	CPUHP_AP_NOTIFY_STARTING,
 	CPUHP_AP_ONLINE,
 	CPUHP_TEARDOWN_CPU,
-	CPUHP_CPU_SET_ACTIVE,
-	CPUHP_KICK_AP_THREAD,
-	CPUHP_BP_ONLINE,
 	CPUHP_AP_ONLINE_IDLE,
 	CPUHP_AP_SMPBOOT_THREADS,
 	CPUHP_AP_NOTIFY_ONLINE,
@@ -86,4 +83,10 @@ static inline void cpuhp_remove_state_no
 	__cpuhp_remove_state(state, false);
 }
 
+#ifdef CONFIG_SMP
+void cpuhp_online_idle(enum cpuhp_state state);
+#else
+static inline void cpuhp_online_idle(enum cpuhp_state state) { }
+#endif
+
 #endif
Index: b/kernel/cpu.c
===================================================================
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -329,6 +329,14 @@ static int notify_starting(unsigned int
 	return 0;
 }
 
+static int bringup_wait_for_ap(unsigned int cpu)
+{
+	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
+
+	wait_for_completion(&st->done);
+	return st->result;
+}
+
 static int bringup_cpu(unsigned int cpu)
 {
 	struct task_struct *idle = idle_thread_get(cpu);
@@ -340,8 +348,9 @@ static int bringup_cpu(unsigned int cpu)
 		cpu_notify(CPU_UP_CANCELED, cpu);
 		return ret;
 	}
+	ret = bringup_wait_for_ap(cpu);
 	BUG_ON(!cpu_online(cpu));
-	return 0;
+	return ret;
 }
 
 /*
@@ -470,7 +479,7 @@ static void cpuhp_thread_fun(unsigned in
 		}
 	} else {
 		/* Cannot happen .... */
-		BUG_ON(st->state < CPUHP_KICK_AP_THREAD);
+		BUG_ON(st->state < CPUHP_AP_ONLINE_IDLE);
 
 		/* Regular hotplug work */
 		if (st->state < st->target)
@@ -780,7 +789,7 @@ static int __ref _cpu_down(unsigned int
 	 * If the current CPU state is in the range of the AP hotplug thread,
 	 * then we need to kick the thread.
 	 */
-	if (st->state >= CPUHP_KICK_AP_THREAD) {
+	if (st->state > CPUHP_TEARDOWN_CPU) {
 		ret = cpuhp_kick_ap_work(cpu);
 		/*
 		 * The AP side has done the error rollback already. Just
@@ -793,11 +802,11 @@ static int __ref _cpu_down(unsigned int
 		 * We might have stopped still in the range of the AP hotplug
 		 * thread. Nothing to do anymore.
 		 */
-		if (st->state >= CPUHP_KICK_AP_THREAD)
+		if (st->state > CPUHP_TEARDOWN_CPU)
 			goto out;
 	}
 	/*
-	 * The AP brought itself down below CPUHP_KICK_AP_THREAD. So we need
+	 * The AP brought itself down to CPUHP_TEARDOWN_CPU. So we need
 	 * to do the further cleanups.
 	 */
 	ret = cpuhp_down_callbacks(cpu, st, cpuhp_bp_states, target);
@@ -859,18 +868,32 @@ void notify_cpu_starting(unsigned int cp
 
 /*
  * Called from the idle task. We need to set active here, so we can kick off
- * the stopper thread.
+ * the stopper thread and unpark the smpboot threads. If the target state is
+ * beyond CPUHP_AP_ONLINE_IDLE we kick cpuhp thread and let it bring up the
+ * cpu further.
  */
-static int cpuhp_set_cpu_active(unsigned int cpu)
+void cpuhp_online_idle(enum cpuhp_state state)
 {
-	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
+	struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);
+	unsigned int cpu = smp_processor_id();
+
+	/* Happens for the boot cpu */
+	if (state != CPUHP_AP_ONLINE_IDLE)
+		return;
+
+	st->state = CPUHP_AP_ONLINE_IDLE;
 
 	/* The cpu is marked online, set it active now */
 	set_cpu_active(cpu, true);
-	/* Unpark the stopper thread and the hotplug thread */
+	/* Unpark the stopper thread and the hotplug thread of this cpu */
 	stop_machine_unpark(cpu);
 	kthread_unpark(st->thread);
-	return 0;
+
+	/* Should we go further up ? */
+	if (st->target > CPUHP_AP_ONLINE_IDLE)
+		__cpuhp_kick_ap_work(st);
+	else
+		complete(&st->done);
 }
 
 /* Requires cpu_add_remove_lock to be held */
@@ -910,7 +933,7 @@ static int _cpu_up(unsigned int cpu, int
 	 * If the current CPU state is in the range of the AP hotplug thread,
 	 * then we need to kick the thread once more.
 	 */
-	if (st->state >= CPUHP_KICK_AP_THREAD) {
+	if (st->state > CPUHP_BRINGUP_CPU) {
 		ret = cpuhp_kick_ap_work(cpu);
 		/*
 		 * The AP side has done the error rollback already. Just
@@ -922,10 +945,10 @@ static int _cpu_up(unsigned int cpu, int
 
 	/*
 	 * Try to reach the target state. We max out on the BP at
-	 * CPUHP_KICK_AP_THREAD. After that the AP hotplug thread is
+	 * CPUHP_BRINGUP_CPU. After that the AP hotplug thread is
 	 * responsible for bringing it up to the target state.
 	 */
-	target = min((int)target, CPUHP_KICK_AP_THREAD);
+	target = min((int)target, CPUHP_BRINGUP_CPU);
 	ret = cpuhp_up_callbacks(cpu, st, cpuhp_bp_states, target);
 out:
 	cpu_hotplug_done();
@@ -1146,22 +1169,7 @@ static struct cpuhp_step cpuhp_bp_states
 		.teardown		= takedown_cpu,
 		.cant_stop		= true,
 	},
-	[CPUHP_CPU_SET_ACTIVE] = {
-		.name			= "cpu:active",
-		.startup		= cpuhp_set_cpu_active,
-		.teardown		= NULL,
-	},
-	[CPUHP_KICK_AP_THREAD] = {
-		.name			= "cpuhp:kickthread",
-		.startup		= cpuhp_kick_ap_work,
-		.teardown		= cpuhp_kick_ap_work,
-	},
 #endif
-	[CPUHP_BP_ONLINE] = {
-		.name			= "online",
-		.startup		= NULL,
-		.teardown		= NULL,
-	},
 };
 
 /* Application processor state steps */
@@ -1204,7 +1212,7 @@ static bool cpuhp_is_ap_state(enum cpuhp
 {
 	if (state >= CPUHP_AP_OFFLINE && state <= CPUHP_AP_ONLINE)
 		return true;
-	return state > CPUHP_BP_ONLINE;
+	return state > CPUHP_BRINGUP_CPU;
 }
 
 static struct cpuhp_step *cpuhp_get_step(enum cpuhp_state state)
Index: b/kernel/sched/idle.c
===================================================================
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -4,6 +4,7 @@
 #include <linux/sched.h>
 #include <linux/cpu.h>
 #include <linux/cpuidle.h>
+#include <linux/cpuhotplug.h>
 #include <linux/tick.h>
 #include <linux/mm.h>
 #include <linux/stackprotector.h>
@@ -276,5 +277,6 @@ static void cpu_idle_loop(void)
 void cpu_startup_entry(enum cpuhp_state state)
 {
 	arch_cpu_idle_prepare();
+	cpuhp_online_idle(state);
 	cpu_idle_loop();
 }