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

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Date:	Tue, 19 Jul 2011 16:39:29 -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>,
	John Stultz <johnstul@...ibm.com>
Subject: Re: [PATCH 2/2] sched: Fix "divide error: 0000" in find_busiest_group



On 07/19/2011 04:33 PM, Peter Zijlstra wrote:
> On Tue, 2011-07-19 at 16:21 -0600, Terry Loftin wrote:
>>> 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.
> 
> Its supposed to roll over on the full 64bit, and I think x86_64 only
> suffers this if you have sched_clock_stable set to 1.
> 
> So I think the correct fix is disabling that logic for now. John Stultz
> was working on some patches to fix __cycles_2_ns().
> 
> Something like the below perhaps.
OK - this would also avoid the problem I came across.  Thank you
for your time.

-T
> 
> ---
>  arch/x86/kernel/cpu/intel.c |    2 --
>  1 files changed, 0 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index 1edf5ba..dba0482 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -91,8 +91,6 @@ static void __cpuinit early_init_intel(struct cpuinfo_x86 *c)
>  	if (c->x86_power & (1 << 8)) {
>  		set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
>  		set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
> -		if (!check_tsc_unstable())
> -			sched_clock_stable = 1;
>  	}
>  
>  	/*
> 
> 
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