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[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>
hifive-unleashed-5.1
Nick Piggin 2007-02-12 00:53:51 -08:00 committed by Linus Torvalds
parent 0a9ac38246
commit ff91691bcc
1 changed files with 5 additions and 3 deletions
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8
kernel/sched.c
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@ -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;
}
}