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Subject[patch 67/83] sched: fix a bug in sched domain degenerate
2.6.27-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Li Zefan <lizf@cn.fujitsu.com>

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@cn.fujitsu.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Gregory Haskins <ghaskins@novell.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
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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