Re: time jumps (again)

From: george anzinger (george@mvista.com)
Date: Mon Aug 04 2003 - 17:38:11 EST


Tim Schmielau wrote:
>>Some days ago I started noticing strange time jumps on my Athlon system.
>>(Asus board, VIA chipset, AMD Athlon 650MHz processor). I haven't
>>noticed them before and I am pretty sure there weren't any for the last
>>few years! Uptime of the machine is now 218 days, and problems began
>>appearing after 215 days approximately.
>>
>>What happens: when doing a
>> $ while true; do date; done
>>I'm noticing time jumps _exactly_ at the beginning of a "new" second (or
>>at the end of an "old" one). the jump is exactly 4294 (4295) seconds
>>into the future. Example:
>>...
>>Mon Aug 4 18:11:06 CEST 2003
>>Mon Aug 4 18:11:06 CEST 2003
>>Mon Aug 4 19:22:41 CEST 2003
>>Mon Aug 4 19:22:41 CEST 2003
>>Mon Aug 4 19:22:41 CEST 2003
>>Mon Aug 4 18:11:07 CEST 2003
>>Mon Aug 4 18:11:07 CEST 2003
>>...
>>
>
>
> Wild guess - does the following patch fix it?

And your theory is that wall_jiffies > jiffies. How does this happen?
  Both of these are only changed under the write_irq lock....

I would feel better with a patch that made jiffies volatile, but it
already is.

I agree that the jump implies overflow here, but just HOW is it happening?

Time for some dianostic code...

  Tim
>
>
> --- linux-2.4.20/arch/i386/kernel/time.c.orig Mon Aug 4 23:38:47 2003
> +++ linux-2.4.20/arch/i386/kernel/time.c Mon Aug 4 23:40:53 2003
> @@ -274,8 +274,8 @@
> read_lock_irqsave(&xtime_lock, flags);
> usec = do_gettimeoffset();
> {
> - unsigned long lost = jiffies - wall_jiffies;
> - if (lost)
> + long lost = jiffies - wall_jiffies;
> + if (lost>0)
> usec += lost * (1000000 / HZ);
> }
> sec = xtime.tv_sec;
>
> -
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.tux.org/lkml/
>

-- 
George Anzinger   george@mvista.com
High-res-timers:  http://sourceforge.net/projects/high-res-timers/
Preemption patch: http://www.kernel.org/pub/linux/kernel/people/rml

- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/



This archive was generated by hypermail 2b29 : Thu Aug 07 2003 - 22:00:26 EST