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>

1 files changed, 12 insertions, 2 deletions

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index bad7dbd4c2e3..444c8a303963 100644
--- a/kernel/rcu/update.c
+++ b/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);
         }
 }