[PATCH 1/2] sched: Prevent divide by zero when cpu_power is 0

[Stefan Bader]

From: Andrew Dickinson <whydna at whydna.net>

This is a patch to fix the corner case where we're crashing with
divide_error in find_busiest_group.

I don't fully understand what the case is that causes sds.total_pwr to
be zero in find_busiest_group, but this patch guards against the
divide-by-zero bug.

I also added safe-guarding around other routines in the scheduler code
where we're dividing by power; that's more of a just-in-case and I'm
definitely open for debate on that.

BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=16991
BugLink: http://bugs.launchpad.net/bugs/614853

Signed-off-by: Andrew Dickinson <whydna at whydna.net>
Signed-off-by: Stefan Bader <stefan.bader at canonical.com>
---
 kernel/sched.c      |   10 +++++++---
 kernel/sched_fair.c |    4 +++-
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 7dd8aad..d4a4b14 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3836,7 +3836,9 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
 	}
 
 	/* Adjust by relative CPU power of the group */
-	sgs->avg_load = (sgs->group_load * SCHED_LOAD_SCALE) / group->cpu_power;
+	sgs->avg_load = (sgs->group_load * SCHED_LOAD_SCALE);
+	if (group->cpu_power)
+		sgs->avg_load /= group->cpu_power;
 
 	/*
 	 * Consider the group unbalanced when the imbalance is larger
@@ -4119,7 +4121,7 @@ find_busiest_group(struct sched_domain *sd, int this_cpu,
 	if (balance && !(*balance))
 		goto ret;
 
-	if (!sds.busiest || sds.busiest_nr_running == 0)
+	if (!sds.busiest || sds.busiest_nr_running == 0 || sds.total_pwr == 0)
 		goto out_balanced;
 
 	if (sds.this_load >= sds.max_load)
@@ -4184,7 +4186,9 @@ find_busiest_queue(struct sched_group *group, enum cpu_idle_type idle,
 		 * the load can be moved away from the cpu that is potentially
 		 * running at a lower capacity.
 		 */
-		wl = (wl * SCHED_LOAD_SCALE) / power;
+		wl = (wl * SCHED_LOAD_SCALE);
+		if (power)
+			wl /= power;
 
 		if (wl > max_load) {
 			max_load = wl;
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 01e311e..3087249 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1344,7 +1344,9 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
 		}
 
 		/* Adjust by relative CPU power of the group */
-		avg_load = (avg_load * SCHED_LOAD_SCALE) / group->cpu_power;
+		avg_load = (avg_load * SCHED_LOAD_SCALE);
+		if (group->cpu_power)
+			avg_load /= group->cpu_power;
 
 		if (local_group) {
 			this_load = avg_load;
-- 
1.7.0.4