(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 some sched domains may be skipped unintentionally when
doing sched domain degenerating.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
---
 kernel/sched.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index dee79ad..b13f45a 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -6875,15 +6875,17 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
 	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)) {
-- 
1.5.4.rc3

--

[ message continues ]

applied to tip/sched/urgent, thanks!

	Ingo
--

[ message continues ]

Hi Ingo,

I just read the modified changelog in the git-log, and it is
wrong (or maybe my fix is wrong?), I should have explained
the bug clearer. :(

I'm writing this mail to confirm if my thought and fix is



And this is right. CPU domain has only 1 cpu so it does not contribute

The bug is, some sched domains won't be checked in the for loop due
to the bug, so they have no chance to be removed.

In the for loop, we check if the parents domains can be removed:

cur_ptr
 |
 v
SMT--->MC--->CPU--->NULL

(parent MC is checked and can be removed)

=>

       cur_ptr
        |
        v
SMT--->CPU--->NULL

(break out of the for loop, because cur_ptr->parent == NULL)

so CPU domain won't be checked. When we delete MC domain, the pointer
should not move forwards, so the fix is:

cur_ptr
 |
 v


--

[ message continues ]

ah, ok - i misunderstood the direction of the fix. So it strengthens 
degeneration - which is a valid fix too. And the commit message 
remains there to shame my reading skills forever ;-)

	Ingo
--

[ message continues ]