On Tue, Nov 12, 2013 at 10:11:26PM +0530, Srikar Dronamraju wrote:
> After Commit-id 863bffc80898 (sched/fair: Fix group power_orig
> computation), we might end up computing group power before the
> sched_domain for a cpu is updated.
>
> Update with cpu_power, if rq->sd is not yet updated.
>
> Signed-off-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
> ---
> Changelog since v1: Fix divide by zero errors that can result because
> power/power_orig was set to 0.
Duh yes!, init_sched_groups_power() is called before we attach the
actual domains, so the above will _always_ fail for the
build_sched_domain() case.
I was a bit puzzled how we could ever have 0 since surely at least the
current cpu should have some !0 contribution, but no!
---
Subject: sched: Check sched_domain before computing group power
From: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
Date: Tue, 12 Nov 2013 22:11:26 +0530
After Commit-id 863bffc80898 (sched/fair: Fix group power_orig
computation), we can dereference rq->sd before its set.
Fix this by falling back to power_of() in this case and add a comment
explaining things.
Cc: mikey@xxxxxxxxxxx
Cc: mingo@xxxxxxxxxx
Cc: hpa@xxxxxxxxx
Cc: tglx@xxxxxxxxxxxxx
Signed-off-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
[peterz: added comment and tweaked patch]
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20131112164126.GF2559@xxxxxxxxxxxxxxxxxx
---
kernel/sched/fair.c | 27 ++++++++++++++++++++++++---
1 file changed, 24 insertions(+), 3 deletions(-)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5354,10 +5354,31 @@ void update_group_power(struct sched_dom
*/
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 (!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 {
/*
--
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