Subject: Re: time jumps forward/backwards

From: Benjamin Gamsa <[email-masked]>
Date: Mon, 31 Aug 2009 20:01:10 -0400

Paul Mackerras wrote:
> Ben Gamsa writes:
> 
>> It appears to be the case that when the time on the system is around the 
>> epoch (1970), that time will occasionally jump forward and then backward 
>> by about 17592 seconds.  When it jumps forward, it always jumps back a 
>> few milliseconds later.  However, it's not always easy to catch these 
>> occurrences.  The delta is more specifically about 17592186059 usec, 
>> give or take a few 10s of microseconds (most of the time), despite the 
>> fact that the user-level program I have that is testing it only checks 
>> every 10 milliseconds.
> 
> I don't think the time code in the kernel is designed to handle
> negative values, i.e., times before the epoch.  If you want it to do
> that you'll have to check places like arch/powerpc/kernel/time.c,
> kernel/time/timekeeping.c, arch/powerpc/include/asm/time.h, etc., and
> make sure that it uses signed types where necessary and that the
> arithmetic is correct.
> 

The time never goes negative.  It starts off at the epoch and moves 
forward, but sometimes it jumps forward by 17952 seconds, and then 
immediately jumps back.  But it never goes negative (or prior to 1970).

	ben