Commit ff91691b authored by Nick Piggin's avatar Nick Piggin Committed by Linus Torvalds

[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: default avatarNick Piggin <npiggin@suse.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 0a9ac382
...@@ -2897,14 +2897,16 @@ static void active_load_balance(struct rq *busiest_rq, int busiest_cpu) ...@@ -2897,14 +2897,16 @@ static void active_load_balance(struct rq *busiest_rq, int busiest_cpu)
static void update_load(struct rq *this_rq) static void update_load(struct rq *this_rq)
{ {
unsigned long this_load; unsigned long this_load;
int i, scale; unsigned int i, scale;
this_load = this_rq->raw_weighted_load; this_load = this_rq->raw_weighted_load;
/* Update our 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; unsigned long old_load, new_load;
/* scale is effectively 1 << i now, and >> i divides by scale */
old_load = this_rq->cpu_load[i]; old_load = this_rq->cpu_load[i];
new_load = this_load; new_load = this_load;
/* /*
...@@ -2914,7 +2916,7 @@ static void update_load(struct rq *this_rq) ...@@ -2914,7 +2916,7 @@ static void update_load(struct rq *this_rq)
*/ */
if (new_load > old_load) if (new_load > old_load)
new_load += scale-1; 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;
} }
} }
......
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