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>

1 files changed, 24 insertions, 3 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e8b652ebe027..fd773ade1a31 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  {
                 /*