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

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Date:	Tue, 19 Jul 2011 16:21:34 -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 2/2] sched: Fix "divide error: 0000" in find_busiest_group



On 07/19/2011 03:18 PM, Peter Zijlstra wrote:
> On Tue, 2011-07-19 at 14:58 -0600, Terry Loftin wrote:
>> Add a check to sched_avg_update() to detect and reset age_stamp if the
>> clock value has wrapped.  Because __cycles_2_ns() includes an offset
>> to account for start up time, the clock may not wrap to zero, so use
>> the current clock value instead.
> 
> So you're running on a platform (unspecified) where we use a raw
> sched_clock() that is buggy. Again, you're fixing symptoms not causes. 
> 
This x86_64.  This is the actual cause, unless the rq->clock
value should never roll, in which case, the clock roll is the
actual cause and you can disregard these patches.

-T
>> Signed-off-by: Terry Loftin <terry.loftin@...com>
>> Signed-off-by: Bob Montgomery <bob.montgomery@...com>
>> ---
>> diff --git a/kernel/sched.c b/kernel/sched.c
>> index 18d38e4..b39cae1 100644
>> --- a/kernel/sched.c
>> +++ b/kernel/sched.c
>> @@ -1256,6 +1256,8 @@ static void sched_avg_update(struct rq *rq)
>>  {
>>  	s64 period = sched_avg_period();
>>
>> +	if (unlikely(rq->age_stamp > rq->clock))
>> +		rq->age_stamp = rq->clock;
>>  	while ((s64)(rq->clock - rq->age_stamp) > period) {
>>  		/*
>>  		 * Inline assembly required to prevent the compiler
> 
> 
> 
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