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

[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

kernel/sched.c

+5 −3
Original line number Diff line number Diff line
@@ -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;

	}

}