linux-kernel - Re: [BUG] sched: clock wrap bug in 2.6.35-stable kills scheduling

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Date:	Mon, 25 Jun 2012 20:49:53 +0200
From:	Thomas Lange <thomas@...elatus.se>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	mingo@...hat.com, gregkh@...uxfoundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [BUG] sched: clock wrap bug in 2.6.35-stable kills scheduling

On 2012-06-25 12:38, Peter Zijlstra wrote:
> On Mon, 2012-06-25 at 11:45 +0200, Peter Zijlstra wrote:
>> On Sun, 2012-06-24 at 19:32 +0200, Thomas Lange wrote:
>>> Bug was introduced in 2.6.35.12 and is still present in linux-2.6.35.y HEAD.
>>
>> Ok, so nobody cares about that.. what does something recent like 3.5-rc4
>> do?

I haven't tested yet. I will try to find the time to do that.

>> If that's fixed, find the patch that fixes it. If not, we'll have a
>> look.

Commit fe44d62 removed the if-clause that caused the problem. Most likely, this
solved the problem.

> 
> 
> If anything, I think something like the below ought to cure things.

I agree. Either we make sure that sched_clock() never wraps or if it wraps,
that it wraps shortly after boot to catch problems early.

> 
> ---
>  kernel/sched/clock.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
> index c685e31..a1a128a 100644
> --- a/kernel/sched/clock.c
> +++ b/kernel/sched/clock.c
> @@ -74,7 +74,7 @@
>   */
>  unsigned long long __attribute__((weak)) sched_clock(void)
>  {
> -	return (unsigned long long)(jiffies - INITIAL_JIFFIES)
> +	return (unsigned long long)(get_jiffies_64() - INITIAL_JIFFIES)
>  					* (NSEC_PER_SEC / HZ);
>  }
>  EXPORT_SYMBOL_GPL(sched_clock);

/Thomas
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