[patch 67/83] sched: fix a bug in sched domain degenerate

[Greg KH]

2.6.27-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Li Zefan <lizf@xxxxxxxxxxxxxx>

commit f29c9b1ccb52904ee442a933cf3dee628f9f4e62 upstream.

Impact: re-add incorrectly eliminated sched domain layers

(1) on i386 with SCHED_SMT and SCHED_MC enabled
	# mount -t cgroup -o cpuset xxx /mnt
	# echo 0 > /mnt/cpuset.sched_load_balance
	# mkdir /mnt/0
	# echo 0 > /mnt/0/cpuset.cpus
	# dmesg
	CPU0 attaching sched-domain:
	 domain 0: span 0 level CPU
	  groups: 0

(2) on i386 with SCHED_MC enabled but SCHED_SMT disabled
	# same with (1)
	# dmesg
	CPU0 attaching NULL sched-domain.

The bug is that some sched domains may be skipped unintentionally when
degenerating (optimizing) sched domains.

Signed-off-by: Li Zefan <lizf@xxxxxxxxxxxxxx>
Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
Cc: Gregory Haskins <ghaskins@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
 kernel/sched.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -6802,15 +6802,17 @@ cpu_attach_domain(struct sched_domain *s
 	struct sched_domain *tmp;
 
 	/* Remove the sched domains which do not contribute to scheduling. */
-	for (tmp = sd; tmp; tmp = tmp->parent) {
+	for (tmp = sd; tmp; ) {
 		struct sched_domain *parent = tmp->parent;
 		if (!parent)
 			break;
+
 		if (sd_parent_degenerate(tmp, parent)) {
 			tmp->parent = parent->parent;
 			if (parent->parent)
 				parent->parent->child = tmp;
-		}
+		} else
+			tmp = tmp->parent;
 	}
 
 	if (sd && sd_degenerate(sd)) {

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