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Commit e0a79f52 authored by Mike Galbraith's avatar Mike Galbraith Committed by Ingo Molnar
Browse files

sched: Fix select_idle_sibling() bouncing cow syndrome 



If the previous CPU is cache affine and idle, select it.

The current implementation simply traverses the sd_llc domain,
taking the first idle CPU encountered, which walks buddy pairs
hand in hand over the package, inflicting excruciating pain.

1 tbench pair (worst case) in a 10 core + SMT package:

  pre   15.22 MB/sec 1 procs
  post 252.01 MB/sec 1 procs

Signed-off-by: default avatarMike Galbraith <bitbucket@online.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1359371965.5783.127.camel@marge.simpson.net


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 60334caf

kernel/sched/fair.c

+7 −14
Original line number Diff line number Diff line
@@ -3252,25 +3252,18 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu) 
 */

static int select_idle_sibling(struct task_struct *p, int target)

{

	int cpu = smp_processor_id();

	int prev_cpu = task_cpu(p);

	struct sched_domain *sd;

	struct sched_group *sg;

	int i;

	int i = task_cpu(p);



	/*

	 * If the task is going to be woken-up on this cpu and if it is

	 * already idle, then it is the right target.

	 */

	if (target == cpu && idle_cpu(cpu))

		return cpu;

	if (idle_cpu(target))

		return target;



	/*

	 * If the task is going to be woken-up on the cpu where it previously

	 * ran and if it is currently idle, then it the right target.

	 * If the prevous cpu is cache affine and idle, don't be stupid.

	 */

	if (target == prev_cpu && idle_cpu(prev_cpu))

		return prev_cpu;

	if (i != target && cpus_share_cache(i, target) && idle_cpu(i))

		return i;



	/*

	 * Otherwise, iterate the domains and find an elegible idle cpu.
@@ -3284,7 +3277,7 @@ static int select_idle_sibling(struct task_struct *p, int target) 
				goto next;



			for_each_cpu(i, sched_group_cpus(sg)) {

				if (!idle_cpu(i))

				if (i == target || !idle_cpu(i))

					goto next;

			}