Re: [patch 2/2] sched: fix select_idle_sibling() logic in select_task_rq_fair()

[Mike Galbraith <efault@gmx.de>]

On Tue, 2010-03-09 at 14:27 +0100, Mike Galbraith wrote:
> On Tue, 2010-03-09 at 07:05 +0100, Mike Galbraith wrote:
> > On Mon, 2010-03-08 at 14:24 -0800, Suresh Siddha wrote:
> 
> > > Ok. I dropped that part in v2 patches that I just posted.
> > 
> > Yeah, I see mails (ramble ramble) crossed in the night.  I'll take them
> > out for a spin, see if your box can get fixed up without busting mine :)
> 
...

BTW, does the below also cure your netbook's woes?  (should)


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@gmx.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@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;