Messages in this thread | | | Subject | Re: gettimeofday() jumping into the future | From | Andi Kleen <> | Date | 31 Mar 2008 09:18:12 +0200 |
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Tim Ricketts <tr@earth.li> writes:
[adding right people to cc just in case it slipped past their filters, keeping enough quote for context]
> On Thu, 23 Aug 2007, Michael Smith wrote: > > > 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. > > I have also seen this. > > > 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. > > Indeed. I don't know where the suggestion of off by 2^32us came in > later in this thread. As you've already pointed out, it's off by > 2^42ns.
[...] > +static inline u64 __get_nsec_offset(void) > { > cycle_t cycle_now, cycle_delta; > - s64 ns_offset; > + u64 ns_offset; > > /* read clocksource: */ > cycle_now = clocksource_read(clock); > > + if (cycle_now < clock->cycle_last) > + return 0;
The old x86-64 pre-clocksource gettimeofday() implementation had a similar check. It came from painful experience.
-Andi
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