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Subject[tip:sched/core] sched: Fix select_idle_sibling() bouncing cow syndrome
Commit-ID:  e0a79f529d5ba2507486d498b25da40911d95cf6
Gitweb: http://git.kernel.org/tip/e0a79f529d5ba2507486d498b25da40911d95cf6
Author: Mike Galbraith <bitbucket@online.de>
AuthorDate: Mon, 28 Jan 2013 12:19:25 +0100
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 4 Feb 2013 20:07:24 +0100

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: Mike 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: Ingo Molnar <mingo@kernel.org>
---
kernel/sched/fair.c | 21 +++++++--------------
1 file changed, 7 insertions(+), 14 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8dbee9f..ed18c74 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -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;
}


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