linux-kernel - Re: [PATCH 1/2] sched: Fix "divide error: 0000" in find_busiest

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Date:	Tue, 19 Jul 2011 16:20:29 -0600
From:	Terry Loftin <terry.loftin@...com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Bob Montgomery <bob.montgomery@...com>
Subject: Re: [PATCH 1/2] sched: Fix "divide error: 0000" in find_busiest_group

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 2.6.32.32, 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 2.6.32.32
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);
 	power >>= SCHED_LOAD_SHIFT;

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