[tip:sched/urgent] sched: Check sched_domain before computing group power

[tip-bot for Srikar Dronamraju <tipbot@zytor.com>]

Commit-ID:  9abf24d465180f5f2eb26a43545348262f16b771
Gitweb:     http://git.kernel.org/tip/9abf24d465180f5f2eb26a43545348262f16b771
Author:     Srikar Dronamraju <srikar@linux.vnet.ibm.com>
AuthorDate: Tue, 12 Nov 2013 22:11:26 +0530
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 19 Nov 2013 17:01:15 +0100

sched: Check sched_domain before computing group power

After commit 863bffc80898 ("sched/fair: Fix group power_orig
computation"), we can dereference rq->sd before it is set.

Fix this by falling back to power_of() in this case and add a comment
explaining things.

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
[ Added comment and tweaked patch. ]
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: mikey@neuling.org
Link: http://lkml.kernel.org/r/20131113151718.GN21461@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/fair.c | 27 ++++++++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e8b652e..fd773ad 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5379,10 +5379,31 @@ void update_group_power(struct sched_domain *sd, int cpu)
 		 */
 
 		for_each_cpu(cpu, sched_group_cpus(sdg)) {
-			struct sched_group *sg = cpu_rq(cpu)->sd->groups;
+			struct sched_group_power *sgp;
+			struct rq *rq = cpu_rq(cpu);
 
-			power_orig += sg->sgp->power_orig;
-			power += sg->sgp->power;
+			/*
+			 * build_sched_domains() -> init_sched_groups_power()
+			 * gets here before we've attached the domains to the
+			 * runqueues.
+			 *
+			 * Use power_of(), which is set irrespective of domains
+			 * in update_cpu_power().
+			 *
+			 * This avoids power/power_orig from being 0 and
+			 * causing divide-by-zero issues on boot.
+			 *
+			 * Runtime updates will correct power_orig.
+			 */
+			if (unlikely(!rq->sd)) {
+				power_orig += power_of(cpu);
+				power += power_of(cpu);
+				continue;
+			}
+
+			sgp = rq->sd->groups->sgp;
+			power_orig += sgp->power_orig;
+			power += sgp->power;
 		}
 	} else  {
 		/*