On Tuesday 30 March 2010 10:08:28 Sebastian Hetze wrote: > On Mon, Mar 29, 2010 at 11:31:13AM +0100, Athanasius wrote: > > On Sun, Mar 28, 2010 at 01:46:35PM +0200, Sebastian Hetze wrote: > > > this message appeared in the KVM guest kern.log last night: > > > > > > Mar 27 22:35:30 guest kernel: [260041.559462] Clocksource tsc unstable > > > (delta = -4398046474878 ns) > > > > > > The guest is running a 2.6.31-20-generic-pae ubuntu kernel with > > > hrtimer-tune-hrtimer_interrupt-hang-logic.patch applied. > > > > > > If I understand things correct, in kernel/time/clocksource.c > > > clocksource_watchdog() checks all the > > > /sys/devices/system/clocksource/clocksource0/available_clocksource > > > every 0.5sec for an delta of more than 0.0625s. So the tsc must have > > > changed more than one hour within two subsequent calls of > > > clocksource_watchdog. No event in the host nor anything in the > > > guest gives reasonable cause for this step. > > > > > > However, the number 4398046474878 is only 36226 ns away from > > > 4*1024*1024*1024*1024 > > > > > I didn't see any such messages but I've had a recent experience with > > > > the time on one KVM host leaping *forwards* approx. 5 and 2.5 hours in > > two separate incidents. Eerily the exact jumps, as best I can tell from > > logs are of 17592 and 8796 seconds, give or take a second or two. If > > you look at these as nanoseconds then that's 'exactly' 2^44 and 2^43 > > nanoseconds. > > > > What I've done that seems to have avoided this happening again is drop > > > > KVM_CLOCK kernel option from the kvm guests' kernel. > > To my understanding, kvm-clock is the best and most reliable clocksource > available, so I do not think it is a good idea to disable it. > > There is a lot of bit shift operation happening with the clocksources, > so there may be a real bug hidden somewhere in the code. > Somehow ntp adjustment is involved, can this cause such huge steps? > Im my case, I actually have NTP running in the guest. However, the > statistics show a pretty stable timing here. > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html I am having the same problem occasional. It only occurs if the VM is under heavy IO or CPU Load but I can't reproduce it 100%. It just never occurs on VMs that only serve a few web pages though. I also noticed that on a machine which has this problem even an ssh shell is *very* laggy so it's not just a cosmetic problem. Would removing the hrtimer from the kernel config solve it or is it necessary for KVM? I remember this problem has been posted her before though there wasn't any real conclusion or solution for it. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
