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    SubjectRe: gettimeofday() jumping into the future
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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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