[PATCH] sched: avoid div in rebalance_tick

Avoid expensive integer divide 3 times per CPU per tick.

A userspace test of this loop went from 26ns, down to 19ns on a G5; and
from 123ns down to 28ns on a P3.

(Also avoid a variable bit shift, as suggested by Alan. The effect
of this wasn't noticable on the CPUs I tested with).

Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 3 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 1fd67e16cd31..08f86178aa34 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2897,14 +2897,16 @@ static void active_load_balance(struct rq *busiest_rq, int busiest_cpu)
 static void update_load(struct rq *this_rq)
 {
 	unsigned long this_load;
-	int i, scale;
+	unsigned int i, scale;
 
 	this_load = this_rq->raw_weighted_load;
 
 	/* Update our load: */
-	for (i = 0, scale = 1; i < 3; i++, scale <<= 1) {
+	for (i = 0, scale = 1; i < 3; i++, scale += scale) {
 		unsigned long old_load, new_load;
 
+		/* scale is effectively 1 << i now, and >> i divides by scale */
+
 		old_load = this_rq->cpu_load[i];
 		new_load = this_load;
 		/*
@@ -2914,7 +2916,7 @@ static void update_load(struct rq *this_rq)
 		 */
 		if (new_load > old_load)
 			new_load += scale-1;
-		this_rq->cpu_load[i] = (old_load*(scale-1) + new_load) / scale;
+		this_rq->cpu_load[i] = (old_load*(scale-1) + new_load) >> i;
 	}
 }