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[ 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 :-) - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||||
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