cpu/hotplug: Make wait for dead cpu completion based

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 <tglx@linutronix.de>
Cc: linux-arch@vger.kernel.org
Cc: Rik van Riel <riel@redhat.com>
Cc: Rafael Wysocki <rafael.j.wysocki@intel.com>
Cc: "Srivatsa S. Bhat" <srivatsa@mit.edu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul Turner <pjt@google.com>
Link: http://lkml.kernel.org/r/20160226182341.776157858@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

4 files changed, 17 insertions, 10 deletions

diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index 83f35767016d..91a48d1b4ca0 100644
--- 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_ */
diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index ad5d7fcb0130..5d68e15e46b7 100644
--- 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,
diff --git a/kernel/cpu.c b/kernel/cpu.c
index f1f880fac832..0e8c07f2566e 100644
--- 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
diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index a4b9813afc96..8abbe89e9114 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -194,8 +194,6 @@ exit_idle:
         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();
                         }