[patch 20/20] timer: Optimization for same expiry time in mod_timer()

[Thomas Gleixner]

From: Anna-Maria Gleixner <anna-maria@xxxxxxxxxxxxx>

The existing optimization for same expiry time in mod_timer() checks whether
the timer expiry time is the same as the new requested expiry time. In the old
timer wheel implementation this does not take the slack batching into account,
neither does the new implementation evaluate whether the new expiry time will
requeue the timer to the same bucket.

To optimize that, we can calculate the resulting bucket and check if the new
expiry time is different from the current expiry time. This calculation
happens outside the base lock held region. If the resulting bucket is the same
we can avoid taking the base lock and requeueing the timer.

If the timer needs to be requeued then we have to check under the base lock
whether the base time has changed between the lockless calculation and taking
the lock. If it has changed we need to recalculate under the lock.

This optimization takes effect for timers which are enqueued into the less
granular wheel levels (1 and above). With a simple test case the functionality
has been verified:

    	    Before	After
Match:	     5.5%	86.6%
Requeue:    94.5%	13.4%
Recalc:  		<0.01%

In the non optimized case the timer is requeued in 94.5% of the cases. With
the index optimization in place the requeue rate drops to 13.4%. The case
where the lockless index calculation has to be redone is less than 0.01%.

With a real world test case (networking) we observed the following changes:

    	    Before	After
Match:	    97.8%	99.7%
Requeue:     2.2%	 0.3%
Recalc:  		<0.001%

That means two percent less lock/requeue/unlock operations in one of the hot
path use cases of timers.

Signed-off-by: Anna-Maria Gleixner <anna-maria@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

---
 kernel/time/timer.c |   51 +++++++++++++++++++++++++++++++++++----------------
 1 file changed, 35 insertions(+), 16 deletions(-)

--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -938,28 +938,36 @@ static inline int
 __mod_timer(struct timer_list *timer, unsigned long expires, bool pending_only)
 {
 	struct timer_base *base, *new_base;
-	unsigned long flags;
+	unsigned int idx = UINT_MAX;
+	unsigned long clk = 0, flags;
 	int ret = 0;
 
 	/*
-	 * TODO: Calculate the array bucket of the timer right here w/o
-	 * holding the base lock. This allows to check not only
-	 * timer->expires == expires below, but also whether the timer
-	 * ends up in the same bucket. If we really need to requeue
-	 * the timer then we check whether base->clk have
-	 * advanced between here and locking the timer base. If
-	 * jiffies advanced we have to recalc the array bucket with the
-	 * lock held.
-	 */
-
-	/*
-	 * This is a common optimization triggered by the
-	 * networking code - if the timer is re-modified
-	 * to be the same thing then just return:
+	 * This is a common optimization triggered by the networking code - if
+	 * the timer is re-modified to be the same thing or ends up in the
+	 * same array bucket then just return:
 	 */
 	if (timer_pending(timer)) {
 		if (timer->expires == expires)
 			return 1;
+		/*
+		 * Take the current timer_jiffies of base, but without holding
+		 * the lock!
+		 */
+		base = get_timer_base(timer->flags);
+		clk = base->clk;
+
+		idx = calc_wheel_index(expires, clk);
+
+		/*
+		 * Retrieve and compare the array index of the pending
+		 * timer. If it matches set the expiry to the new value so a
+		 * subsequent call will exit in the expires check above.
+		 */
+		if (idx == timer_get_idx(timer)) {
+			timer->expires = expires;
+			return 1;
+		}
 	}
 
 	timer_stats_timer_set_start_info(timer);
@@ -996,7 +1004,18 @@ static inline int
 	}
 
 	timer->expires = expires;
-	internal_add_timer(base, timer);
+	/*
+	 * If idx was calculated above and the base time did not advance
+	 * between calculating idx and taking the lock, only enqueue_timer()
+	 * and trigger_dyntick_cpu() is required. Otherwise we need to
+	 * (re)calculate the wheel index via internal_add_timer().
+	 */
+	if (idx != UINT_MAX && clk == base->clk) {
+		enqueue_timer(base, timer, idx);
+		trigger_dyntick_cpu(base, timer);
+	} else {
+		internal_add_timer(base, timer);
+	}
 
 out_unlock:
 	spin_unlock_irqrestore(&base->lock, flags);