[PATCH tip/core/timers 2/4] timers: Reduce __run_timers() latency for empty list

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

The __run_timers() function currently steps through the list one jiffy at
a time in order to update the timer wheel.  However, if the timer wheel
is empty, no adjustment is needed other than updating ->timer_jiffies.
In this case, which is likely to be common for NO_HZ_FULL kernels, the
kernel currently incurs a large latency for no good reason.  This commit
therefore short-circuits this case.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/timer.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/kernel/timer.c b/kernel/timer.c
index 2245b7374c3d..295837e5e011 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -338,6 +338,17 @@ void set_timer_slack(struct timer_list *timer, int slack_hz)
 }
 EXPORT_SYMBOL_GPL(set_timer_slack);
 
+static bool catchup_timer_jiffies(struct tvec_base *base)
+{
+#ifdef CONFIG_NO_HZ_FULL
+	if (!base->all_timers) {
+		base->timer_jiffies = jiffies;
+		return 1;
+	}
+#endif /* #ifdef CONFIG_NO_HZ_FULL */
+	return 0;
+}
+
 static void
 __internal_add_timer(struct tvec_base *base, struct timer_list *timer)
 {
@@ -1150,6 +1161,10 @@ static inline void __run_timers(struct tvec_base *base)
 	struct timer_list *timer;
 
 	spin_lock_irq(&base->lock);
+	if (catchup_timer_jiffies(base)) {
+		spin_unlock_irq(&base->lock);
+		return;
+	}
 	while (time_after_eq(jiffies, base->timer_jiffies)) {
 		struct list_head work_list;
 		struct list_head *head = &work_list;
-- 
1.8.1.5