From: Benjamin Gamsa <ben@somanetworks•com>
To: Paul Mackerras <paulus@samba•org>
Cc: linuxppc-dev@lists•ozlabs.org
Subject: Re: time jumps forward/backwards
Date: Mon, 31 Aug 2009 20:09:49 -0400 [thread overview]
Message-ID: <4A9C664D.6010504@somanetworks.com> (raw)
In-Reply-To: <4A9C6446.2000202@somanetworks.com>
Benjamin Gamsa wrote:
> 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).
>
One important thing I forgot to add is that ntpd is running on this
system, but the ntp servers are not available. I suspect the problem
may be related to ntpd, even though I've seen the time jump even when I
had ntpd stopped within gdb. I've not yet been able to confirm if the
problem still occurs when ntpd is never even started, although I will be
testing that soon (the tests often require many hours to establish if
there are no jumps).
ben
next prev parent reply other threads:[~2009-09-01 0:09 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-08-31 21:53 time jumps forward/backwards Ben Gamsa
2009-08-31 23:45 ` Paul Mackerras
2009-09-01 0:01 ` Benjamin Gamsa
2009-09-01 0:09 ` Benjamin Gamsa [this message]
2009-09-01 2:20 ` Benjamin Gamsa
2009-09-01 2:31 ` Sean MacLennan
2009-09-01 3:57 ` Benjamin Gamsa
2009-09-01 10:49 ` Benjamin Herrenschmidt
2009-09-01 11:20 ` Benjamin Gamsa
2009-09-03 12:49 ` Ben Gamsa
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