[PATCH 1/2] sched: Prevent divide by zero when cpu_power is 0
Andy Whitcroft
apw at canonical.com
Wed Jan 19 14:14:53 UTC 2011
On Tue, Jan 18, 2011 at 04:34:22PM +0100, Stefan Bader wrote:
> 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;
This one looks ok. Seems to avoid doing the divide when it would trip
by zero.
Acked-by: Andy Whitcroft <apw at canonical.com>
-apw
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