Re: [PATCH 1/2] sched: Fix "divide error: 0000" in find_busiest_group

From: Terry Loftin
Date: Tue Jul 19 2011 - 18:20:23 EST

On 07/19/2011 03:17 PM, Peter Zijlstra wrote:
> On Tue, 2011-07-19 at 14:58 -0600, Terry Loftin wrote:
>> Correct the protection expression in update_cpu_power() to avoid setting
>> rq->cpu_power to zero.
> Firstly you fail to mention what kernel this is again, secondly this
> should never happen in the first place, so this fix is wrong. At best it
> papers over another bug.

My Apologies, this was found on kernel, but the all
the related code is the same in v3.0-rc7. The patch is against
v3.0-rc7. I've done some limited testing of this on
by modifying __cycles_2_ns() to add an offset to the TSC when
it is read to simulate 208 days of uptime, but that kernel has
only been running for a couple days.

I also agree this should never happen. As the statement currently
stands, it won't work - so it should either be corrected or removed.
Here is the alternative patch:

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 0c26e2d..f9c9a89 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -2549,9 +2549,6 @@ static void update_cpu_power(struct sched_domain *sd, int cpu)
power *= scale_rt_power(cpu);

- if (!power)
- power = 1;
cpu_rq(cpu)->cpu_power = power;
sdg->cpu_power = power;
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