[PATCH] timer: Lazily wakup nohz CPU when adding new timer.

[Yunhong Jiang]

Currently, when a new timer added to timer wheel for a nohz_active CPU,
the target CPU will always be waked up.

In fact, if the new added timer is after the base->next_timer, we don't
need wake up the target CPU since it will not change the sleep time. A
lazy wake up is better in such scenario.

I cooked a test scenario. On my 32 cores system, a driver on CPU 15
continuous enqueues timer to CPU 8/9/10/11 with random expire and then
checks the idle_calls difference after 10 seconds. Below data shows
that lazy wake up do reduce the wakeup a lot.

		w/o Lazy	w/ lazy
CPU 8:		135		88
CPU 9:		238		43
CPU 10:		157		83
CPU 11:		172		70

Signed-off-by: Yunhong Jiang <yunhong.jiang@xxxxxxxxxxxxxxx>
---
 kernel/time/timer.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index d3f5e92f722a..a039d9e6b55a 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -414,6 +414,8 @@ __internal_add_timer(struct tvec_base *base, struct timer_list *timer)
 
 static void internal_add_timer(struct tvec_base *base, struct timer_list *timer)
 {
+	bool kick_nohz = false;
+
 	/* Advance base->jiffies, if the base is empty */
 	if (!base->all_timers++)
 		base->timer_jiffies = jiffies;
@@ -424,9 +426,17 @@ static void internal_add_timer(struct tvec_base *base, struct timer_list *timer)
 	 */
 	if (!(timer->flags & TIMER_DEFERRABLE)) {
 		if (!base->active_timers++ ||
-		    time_before(timer->expires, base->next_timer))
+		    time_before(timer->expires, base->next_timer)) {
 			base->next_timer = timer->expires;
-	}
+			/*
+			 * CPU in dynticks need reevaluate the timer wheel
+			 * if newer timer added with next_timer updated.
+			 */
+			if (base->nohz_active)
+				kick_nohz = true;
+		}
+	} else if (base->nohz_active && tick_nohz_full_cpu(base->cpu))
+		kick_nohz = true;
 
 	/*
 	 * Check whether the other CPU is in dynticks mode and needs
@@ -441,11 +451,8 @@ static void internal_add_timer(struct tvec_base *base, struct timer_list *timer)
 	 * require special care against races with idle_cpu(), lets deal
 	 * with that later.
 	 */
-	if (base->nohz_active) {
-		if (!(timer->flags & TIMER_DEFERRABLE) ||
-		    tick_nohz_full_cpu(base->cpu))
-			wake_up_nohz_cpu(base->cpu);
-	}
+	if (kick_nohz)
+		wake_up_nohz_cpu(base->cpu);
 }
 
 #ifdef CONFIG_TIMER_STATS
-- 
1.8.3.1

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