rcu: Improve RCU-tasks energy efficiency

The current RCU-tasks implementation uses strict polling to detect
callback arrivals.  This works quite well, but is not so good for
energy efficiency.  This commit therefore replaces the strict polling
with a wait queue.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/update.c

@@ -365,6 +365,7 @@ early_initcall(check_cpu_stall_init);
 /* Global list of callbacks and associated lock. */
 static struct rcu_head *rcu_tasks_cbs_head;
 static struct rcu_head **rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
+static DECLARE_WAIT_QUEUE_HEAD(rcu_tasks_cbs_wq);
 static DEFINE_RAW_SPINLOCK(rcu_tasks_cbs_lock);
 
 /* Track exiting tasks in order to allow them to be waited for. */
@@ -378,13 +379,17 @@ module_param(rcu_task_stall_timeout, int, 0644);
 void call_rcu_tasks(struct rcu_head *rhp, void (*func)(struct rcu_head *rhp))
 {
 	unsigned long flags;
+	bool needwake;
 
 	rhp->next = NULL;
 	rhp->func = func;
 	raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
+	needwake = !rcu_tasks_cbs_head;
 	*rcu_tasks_cbs_tail = rhp;
 	rcu_tasks_cbs_tail = &rhp->next;
 	raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
+	if (needwake)
+		wake_up(&rcu_tasks_cbs_wq);
 }
 EXPORT_SYMBOL_GPL(call_rcu_tasks);
 
@@ -495,8 +500,12 @@ static int __noreturn rcu_tasks_kthread(void *arg)
 
 		/* If there were none, wait a bit and start over. */
 		if (!list) {
-			schedule_timeout_interruptible(HZ);
-			WARN_ON(signal_pending(current));
+			wait_event_interruptible(rcu_tasks_cbs_wq,
+						 rcu_tasks_cbs_head);
+			if (!rcu_tasks_cbs_head) {
+				WARN_ON(signal_pending(current));
+				schedule_timeout_interruptible(HZ/10);
+			}
 			continue;
 		}
 
@@ -602,6 +611,7 @@ static int __noreturn rcu_tasks_kthread(void *arg)
 			list = next;
 			cond_resched();
 		}
+		schedule_timeout_uninterruptible(HZ/10);
 	}
 }