Re: time jumps (again)

[george anzinger <george@mvista.com>]

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;
> 
> -
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-- 
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