[PATCH] Optimize calc_load()

[Eric Dumazet]

calc_load() is called by timer interrupt to update avenrun[].
It currently calls nr_active() at each timer tick (HZ per second), while the 
update of avenrun[] is done only once every 5 seconds. (LOAD_FREQ=5 Hz)

nr_active() is quite expensive on SMP machines, since it has to sum up 
nr_running and nr_uninterruptible of all online CPUS, bringing foreign dirty 
cache lines.

This patch is an optimization of calc_load() so that nr_active() is called 
only if we need it.

The use of unlikely() is welcome since the condition is true only once every 
5*HZ time.

Signed-off-by: Eric Dumazet <dada1@xxxxxxxxxxxxx>
--- linux-2.6.19/kernel/timer.c	2006-12-11 12:27:28.000000000 +0100
+++ linux-2.6.19-ed/kernel/timer.c	2006-12-11 12:29:11.000000000 +0100
@@ -1008,11 +1008,15 @@ static inline void calc_load(unsigned lo
 	unsigned long active_tasks; /* fixed-point */
 	static int count = LOAD_FREQ;
 
-	active_tasks = count_active_tasks();
-	for (count -= ticks; count < 0; count += LOAD_FREQ) {
-		CALC_LOAD(avenrun[0], EXP_1, active_tasks);
-		CALC_LOAD(avenrun[1], EXP_5, active_tasks);
-		CALC_LOAD(avenrun[2], EXP_15, active_tasks);
+	count -= ticks;
+	if (unlikely(count < 0)) {
+		active_tasks = count_active_tasks();
+		do {
+			CALC_LOAD(avenrun[0], EXP_1, active_tasks);
+			CALC_LOAD(avenrun[1], EXP_5, active_tasks);
+			CALC_LOAD(avenrun[2], EXP_15, active_tasks);
+			count += LOAD_FREQ;
+		} while (count < 0);
 	}
 }