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From: Mike Galbraith on 11 Mar 2010 05:00

sched: fix select_idle_sibling()

Don't bother with selection when the current cpu is idle. Recent load
balancing changes also make it no longer necessary to check wake_affine()
success before returning the selected sibling, so we now always use it.

Signed-off-by: Mike Galbraith <efault(a)gmx.de>
Cc: Ingo Molnar <mingo(a)elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra(a)chello.nl>
LKML-Reference: <new-submission>

---
kernel/sched_fair.c | 14 ++++++++++----
1 file changed, 10 insertions(+), 4 deletions(-)

Index: linux-2.6/kernel/sched_fair.c
===================================================================
--- linux-2.6.orig/kernel/sched_fair.c
+++ linux-2.6/kernel/sched_fair.c
@@ -1438,7 +1438,7 @@ static int select_task_rq_fair(struct ta
int cpu = smp_processor_id();
int prev_cpu = task_cpu(p);
int new_cpu = cpu;
- int want_affine = 0;
+ int want_affine = 0, cpu_idle = !current->pid;
int want_sd = 1;
int sync = wake_flags & WF_SYNC;

@@ -1496,13 +1496,15 @@ static int select_task_rq_fair(struct ta
* If there's an idle sibling in this domain, make that
* the wake_affine target instead of the current cpu.
*/
- if (tmp->flags & SD_SHARE_PKG_RESOURCES)
+ if (!cpu_idle && tmp->flags & SD_SHARE_PKG_RESOURCES)
target = select_idle_sibling(p, tmp, target);

if (target >= 0) {
if (tmp->flags & SD_WAKE_AFFINE) {
affine_sd = tmp;
want_affine = 0;
+ if (target != cpu)
+ cpu_idle = 1;
}
cpu = target;
}
@@ -1518,6 +1520,7 @@ static int select_task_rq_fair(struct ta
sd = tmp;
}

+#ifdef CONFIG_FAIR_GROUP_SCHED
if (sched_feat(LB_SHARES_UPDATE)) {
/*
* Pick the largest domain to update shares over
@@ -1531,9 +1534,12 @@ static int select_task_rq_fair(struct ta
if (tmp)
update_shares(tmp);
}
+#endif

- if (affine_sd && wake_affine(affine_sd, p, sync))
- return cpu;
+ if (affine_sd) {
+ if (cpu_idle || cpu == prev_cpu || wake_affine(affine_sd, p, sync))
+ return cpu;
+ }

while (sd) {
int load_idx = sd->forkexec_idx;


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Prev: sched: remove FAIR_SLEEPERS feature
Next: sched: tweak sched_latency and min_granularity