[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;
         }
 }