[PATCH RT 28/31] timers: Reduce __run_timers() latency for empty list

[Steven Rostedt]

3.10.70-rt75-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>

upstream commit: d550e81dc0ddc04f1b417c179c214103a28e0ee8

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@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Reviewed-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Reviewed-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Tested-by: Mike Galbraith <bitbucket@xxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 kernel/timer.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/kernel/timer.c b/kernel/timer.c
index 23919c7c212f..c20bd12a4f06 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -341,6 +341,20 @@ void set_timer_slack(struct timer_list *timer, int slack_hz)
 }
 EXPORT_SYMBOL_GPL(set_timer_slack);
 
+/*
+ * If the list is empty, catch up ->timer_jiffies to the current time.
+ * The caller must hold the tvec_base lock.  Returns true if the list
+ * was empty and therefore ->timer_jiffies was updated.
+ */
+static bool catchup_timer_jiffies(struct tvec_base *base)
+{
+	if (!base->all_timers) {
+		base->timer_jiffies = jiffies;
+		return true;
+	}
+	return false;
+}
+
 static void
 __internal_add_timer(struct tvec_base *base, struct timer_list *timer)
 {
@@ -1203,6 +1217,10 @@ 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;
-- 
2.1.4


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