[PATCH] hrtimers: optimize softirq runqueues

The hrtimer softirq is called from the timer softirq every tick.  Retrieve the
current time from xtime and wall_to_monotonic instead of calling
base->get_time() for each timer base.  Store the time in the base structure
and provide a hook once clock source abstractions are in place and to keep the
code open for new base clocks.

Based on a patch from: Roman Zippel <zippel@linux-m68k.org>

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/hrtimer.h

@@ -72,14 +72,16 @@ struct hrtimer {
 /**
  * struct hrtimer_base - the timer base for a specific clock
  *
- * @index:	clock type index for per_cpu support when moving a timer
- *		to a base on another cpu.
- * @lock:	lock protecting the base and associated timers
- * @active:	red black tree root node for the active timers
- * @first:	pointer to the timer node which expires first
- * @resolution:	the resolution of the clock, in nanoseconds
- * @get_time:	function to retrieve the current time of the clock
- * @curr_timer:	the timer which is executing a callback right now
+ * @index:		clock type index for per_cpu support when moving a timer
+ *			to a base on another cpu.
+ * @lock:		lock protecting the base and associated timers
+ * @active:		red black tree root node for the active timers
+ * @first:		pointer to the timer node which expires first
+ * @resolution:		the resolution of the clock, in nanoseconds
+ * @get_time:		function to retrieve the current time of the clock
+ * @get_sofirq_time:	function to retrieve the current time from the softirq
+ * @curr_timer:		the timer which is executing a callback right now
+ * @softirq_time:	the time when running the hrtimer queue in the softirq
  */
 struct hrtimer_base {
 	clockid_t		index;
@@ -88,7 +90,9 @@ struct hrtimer_base {
 	struct rb_node		*first;
 	ktime_t			resolution;
 	ktime_t			(*get_time)(void);
+	ktime_t			(*get_softirq_time)(void);
 	struct hrtimer		*curr_timer;
+	ktime_t			softirq_time;
 };
 
 /*

kernel/hrtimer.c

@@ -122,6 +122,26 @@ void ktime_get_ts(struct timespec *ts)
 }
 EXPORT_SYMBOL_GPL(ktime_get_ts);
 
+/*
+ * Get the coarse grained time at the softirq based on xtime and
+ * wall_to_monotonic.
+ */
+static void hrtimer_get_softirq_time(struct hrtimer_base *base)
+{
+	ktime_t xtim, tomono;
+	unsigned long seq;
+
+	do {
+		seq = read_seqbegin(&xtime_lock);
+		xtim = timespec_to_ktime(xtime);
+		tomono = timespec_to_ktime(wall_to_monotonic);
+
+	} while (read_seqretry(&xtime_lock, seq));
+
+	base[CLOCK_REALTIME].softirq_time = xtim;
+	base[CLOCK_MONOTONIC].softirq_time = ktime_add(xtim, tomono);
+}
+
 /*
  * Functions and macros which are different for UP/SMP systems are kept in a
  * single place
@@ -586,9 +606,11 @@ int hrtimer_get_res(const clockid_t which_clock, struct timespec *tp)
  */
 static inline void run_hrtimer_queue(struct hrtimer_base *base)
 {
-	ktime_t now = base->get_time();
 	struct rb_node *node;
 
+	if (base->get_softirq_time)
+		base->softirq_time = base->get_softirq_time();
+
 	spin_lock_irq(&base->lock);
 
 	while ((node = base->first)) {
@@ -598,7 +620,7 @@ static inline void run_hrtimer_queue(struct hrtimer_base *base)
 		void *data;
 
 		timer = rb_entry(node, struct hrtimer, node);
-		if (now.tv64 <= timer->expires.tv64)
+		if (base->softirq_time.tv64 <= timer->expires.tv64)
 			break;
 
 		fn = timer->function;
@@ -641,6 +663,8 @@ void hrtimer_run_queues(void)
 	struct hrtimer_base *base = __get_cpu_var(hrtimer_bases);
 	int i;
 
+	hrtimer_get_softirq_time(base);
+
 	for (i = 0; i < MAX_HRTIMER_BASES; i++)
 		run_hrtimer_queue(&base[i]);
 }