linux-kernel - [PATCH tip/core/timers 1/3] timers: Reduce __run

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Message-Id: <1389672919-14621-1-git-send-email-paulmck@linux.vnet.ibm.com>
Date:	Mon, 13 Jan 2014 20:15:17 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, fweisbec@...il.com,
	oleg@...hat.com, sbw@....edu,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/timers 1/3] timers: Reduce __run_timers() latency for empty list

From: "Paul E. McKenney" <paulmck@...ux.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@...ux.vnet.ibm.com>
---
 kernel/timer.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/kernel/timer.c b/kernel/timer.c
index 6582b82fa966..21849275828f 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -337,6 +337,17 @@ 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->active_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)
 {
@@ -1146,6 +1157,10 @@ 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

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