Messages in this thread | | | Date | Thu, 23 Aug 2007 14:20:58 +0200 | From | "Michael Smith" <> | Subject | Re: gettimeofday() jumping into the future |
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On 8/23/07, Peter Zijlstra <peterz@infradead.org> wrote: > [ 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 :-)
My apologies. It's always the obvious things one forgets to put in...
This was originally seen on an FC5 system running: 2.6.18-1.2257.fc5smp
We've reproduced it on the same system with a kernel built from linus's git tree yesterday: 2.6.23-rc3.
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