[patch 19/20] cpu/hotplug: Make wait for dead cpu completion based

[Thomas Gleixner]

Kill the busy spinning on the control side and just wait for the hotplugged
cpu to tell that it reached the dead state.

Signed-off-by: Thomas Gleixner <t...@linutronix.de>
---
 include/linux/cpu.h        |    5 +++--
 include/linux/cpuhotplug.h |    1 +
 kernel/cpu.c               |   16 ++++++++++++----
 kernel/sched/idle.c        |    5 +----
 4 files changed, 17 insertions(+), 10 deletions(-)

Index: b/include/linux/cpu.h
===================================================================
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -276,14 +276,15 @@ void arch_cpu_idle_enter(void);
 void arch_cpu_idle_exit(void);
 void arch_cpu_idle_dead(void);
 
-DECLARE_PER_CPU(bool, cpu_dead_idle);
-
 int cpu_report_state(int cpu);
 int cpu_check_up_prepare(int cpu);
 void cpu_set_state_online(int cpu);
 #ifdef CONFIG_HOTPLUG_CPU
 bool cpu_wait_death(unsigned int cpu, int seconds);
 bool cpu_report_death(void);
+void cpuhp_report_idle_dead(void);
+#else
+static inline void cpuhp_report_idle_dead(void) { }
 #endif /* #ifdef CONFIG_HOTPLUG_CPU */
 
 #endif /* _LINUX_CPU_H_ */
Index: b/include/linux/cpuhotplug.h
===================================================================
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -6,6 +6,7 @@ enum cpuhp_state {
        CPUHP_CREATE_THREADS,
        CPUHP_NOTIFY_PREPARE,
        CPUHP_BRINGUP_CPU,
+       CPUHP_AP_IDLE_DEAD,
        CPUHP_AP_OFFLINE,
        CPUHP_AP_NOTIFY_STARTING,
        CPUHP_AP_ONLINE,
Index: b/kernel/cpu.c
===================================================================
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -688,6 +688,7 @@ static int take_cpu_down(void *_param)
 
 static int takedown_cpu(unsigned int cpu)
 {
+       struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
        int err;
 
        /*
@@ -733,10 +734,8 @@ static int takedown_cpu(unsigned int cpu
         *
         * Wait for the stop thread to go away.
         */
-       while (!per_cpu(cpu_dead_idle, cpu))
-               cpu_relax();
-       smp_mb(); /* Read from cpu_dead_idle before __cpu_die(). */
-       per_cpu(cpu_dead_idle, cpu) = false;
+       wait_for_completion(&st->done);
+       BUG_ON(st->state != CPUHP_AP_IDLE_DEAD);
 
        /* Interrupts are moved away from the dying cpu, reenable alloc/free */
        irq_unlock_sparse();
@@ -756,6 +755,15 @@ static int notify_dead(unsigned int cpu)
        return 0;
 }
 
+void cpuhp_report_idle_dead(void)
+{
+       struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);
+
+       BUG_ON(st->state != CPUHP_AP_OFFLINE);
+       st->state = CPUHP_AP_IDLE_DEAD;
+       complete(&st->done);
+}
+
 #else
 #define notify_down_prepare    NULL
 #define takedown_cpu           NULL
Index: b/kernel/sched/idle.c
===================================================================
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -194,8 +194,6 @@ static void cpuidle_idle_call(void)
        rcu_idle_exit();
 }
 
-DEFINE_PER_CPU(bool, cpu_dead_idle);
-
 /*
  * Generic idle loop implementation
  *
@@ -224,8 +222,7 @@ static void cpu_idle_loop(void)
                        if (cpu_is_offline(smp_processor_id())) {
                                rcu_cpu_notify(NULL, CPU_DYING_IDLE,
                                               (void 
*)(long)smp_processor_id());
-                               smp_mb(); /* all activity before dead. */
-                               this_cpu_write(cpu_dead_idle, true);
+                               cpuhp_report_idle_dead();
                                arch_cpu_idle_dead();
                        }