Re: gettimeofday() jumping into the future

Peter Zijlstra
2007-08-23T11:47:32Z

[ CCs added ]

On Thu, 2007-08-23 at 13:08 +0200, Michael Smith wrote:
> Hi,
>
> We've been seeing some strange behaviour on some of our applications
> recently. I've tracked this down to gettimeofday() returning spurious
> values occasionally.
>
> Specifically, gettimeofday() will suddenly, for a single call, return
> a value about 4398 seconds (~1 hour 13 minutes) in the future. The
> following call goes back to a normal value.
>
> This seems to be occurring when the clock source goes slightly
> backwards for a single call. In
> kernel/time/timekeeping.c:__get_nsec_offset(), we have this:
> cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;
>
> So a small decrease in time here will (this is all unsigned
> arithmetic) give us a very large cycle_delta. cyc2ns() then multiplies
> this by some value, then right shifts by 22. The resulting value (in
> nanoseconds) is approximately 4398 seconds; this gets added on to the
> xtime value, giving us our jump into the future. The next call to
> gettimeofday() returns to normal as we don't have this huge nanosecond
> offset.
>
> This system is a 2-socket core 2 quad machine (8 cpus), running 32 bit
> mode. It's a dell poweredge 1950. The kernel selects the TSC as the
> clock source, having determined that the tsc runs synchronously on
> this system. Switching the systems to use a different time source
> seems to make the problem go away (which is fine for us, but we'd like
> to get this fixed properly upstream).
>
> We've also seen this behaviour with a synthetic test program (which
> just runs 4 threads all calling gettimeofday() in a loop as fast as
> possible and testing that it doesn't jump) on an older machine, a dell
> poweredge SC1425 with two p4 hyperthreaded xeons.
>
> Can anyone advise on what's going wrong here? I can't find much in the
> way of documentation on whether the TSC is guaranteed to be
> monotonically increasing on intel systems. Should the code choose not
> to use the TSC? Or should the TSC reading code ensure that the
> returned values are monotonic?
>
> Is there any more information that would be useful? I'll be on a plane
> for most of tomorrow, so might be a little slow responding.

The exact version of the kernel you're using might be good thing to
start with :-)