Am 15.09.2010 01:40, Zachary Amsden wrote: > On 09/14/2010 12:26 PM, Jan Kiszka wrote: >> Am 14.09.2010 21:32, Zachary Amsden wrote: >> >>> On 09/14/2010 12:40 AM, Jan Kiszka wrote: >>> >>>> Am 14.09.2010 11:27, Avi Kivity wrote: >>>> >>>> >>>>> On 09/14/2010 11:10 AM, Jan Kiszka wrote: >>>>> >>>>> >>>>>> Am 20.08.2010 10:07, Zachary Amsden wrote: >>>>>> >>>>>> >>>>>>> When CPUs with unstable TSCs enter deep C-state, TSC may stop >>>>>>> running. This causes us to require resynchronization. Since >>>>>>> we can't tell when this may potentially happen, we assume the >>>>>>> worst by forcing re-compensation for it at every point the VCPU >>>>>>> task is descheduled. >>>>>>> >>>>>>> Signed-off-by: Zachary Amsden<zamsden@xxxxxxxxxx> >>>>>>> --- >>>>>>> arch/x86/kvm/x86.c | 2 +- >>>>>>> 1 files changed, 1 insertions(+), 1 deletions(-) >>>>>>> >>>>>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >>>>>>> index 7fc4a55..52b6c21 100644 >>>>>>> --- a/arch/x86/kvm/x86.c >>>>>>> +++ b/arch/x86/kvm/x86.c >>>>>>> @@ -1866,7 +1866,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu >>>>>>> *vcpu, int cpu) >>>>>>> } >>>>>>> >>>>>>> kvm_x86_ops->vcpu_load(vcpu, cpu); >>>>>>> - if (unlikely(vcpu->cpu != cpu)) { >>>>>>> + if (unlikely(vcpu->cpu != cpu) || check_tsc_unstable()) { >>>>>>> /* Make sure TSC doesn't go backwards */ >>>>>>> s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 : >>>>>>> native_read_tsc() - vcpu->arch.last_host_tsc; >>>>>>> >>>>>>> >>>>>> For yet unknown reason, this commit breaks Linux guests here if they >>>>>> are >>>>>> started with only a single VCPU. They hang during boot, obviously no >>>>>> longer receiving interrupts. >>>>>> >>>>>> I'm using kvm-kmod against a 2.6.34 host kernel, so this may be a >>>>>> side >>>>>> effect of the wrapping, though I cannot imagine how. >>>>>> >>>>>> Anyone any ideas? >>>>>> >>>>>> >>>>>> >>>>>> >>>>> Most likely, time went backwards, and some 'future - past' calculation >>>>> resulted in a negative sleep value which was then interpreted as >>>>> unsigned and resulted in a 2342525634 year sleep. >>>>> >>>>> >>>> Looks like that's the case on first glance at the apic state. >>>> >>>> >>> This compensation effectively nulls the delta between current and >>> last TSC: >>> >>> if (unlikely(vcpu->cpu != cpu) || check_tsc_unstable()) { >>> /* Make sure TSC doesn't go backwards */ >>> s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 : >>> native_read_tsc() - >>> vcpu->arch.last_host_tsc; >>> if (tsc_delta< 0) >>> mark_tsc_unstable("KVM discovered backwards >>> TSC"); >>> if (check_tsc_unstable()) >>> kvm_x86_ops->adjust_tsc_offset(vcpu, >>> -tsc_delta); >>> kvm_migrate_timers(vcpu); >>> vcpu->cpu = cpu; >>> >>> If TSC has advanced quite a bit due to a TSC jump during sleep(*), it >>> will adjust the offset backwards to compensate; similarly, if it has >>> gone backwards, it will advance the offset. >>> >>> In neither case should the visible TSC go backwards, assuming >>> last_host_tsc is recorded properly, and so kvmclock should be similarly >>> unaffected. >>> >>> Perhaps the guest is more intelligent than we hope, and is comparing two >>> different clocks: kvmclock or TSC with the rate of PIT interrupts. This >>> could result in negative arithmetic begin interpreted as unsigned. Are >>> you using PIT interrupt reinjection on this guest or passing >>> -no-kvm-pit-reinjection? >>> >>> >>>> >>>> >>>>> Does your guest use kvmclock, tsc, or some other time source? >>>>> >>>>> >>>> A kernel that has kvmclock support even hangs in SMP mode. The others >>>> pick hpet or acpi_pm. TSC is considered unstable. >>>> >>>> >>> SMP mode here has always and will always be unreliable. Are you running >>> on an Intel or AMD CPU? The origin of this code comes from a workaround >>> for (*) in vendor-specific code, and perhaps it is inappropriate for >>> both. >>> >> I'm on a fairly new Intel i7 (M 620). And I accidentally rebooted my box >> a few hours ago. Well, the issue is gone now... >> >> So I looked into the system logs and found this: >> >> [18446744053.434939] PM: resume of devices complete after 4379.595 msecs >> [18446744053.457133] PM: Finishing wakeup. >> [18446744053.457135] Restarting tasks ... >> [ 0.000999] Marking TSC unstable due to KVM discovered backwards TSC >> [270103.974668] done. >> >> From that point on the box was on hpet, including the time I did the >> failing tests this morning. The kvm-kmod version loaded at this point >> was based on kvm.git df549cfc. >> >> But my /proc/cpuinfo claims "constant_tsc", and Linux is generally happy >> with using it as clock source. Does this tell you anything? >> > > Yes, quite a bit. > > It's possible that marking the TSC unstable with an actively running VM > causes a boundary condition that I had not accounted for. It's also > possible that the clocksource switch triggered some bad behavior. Suspend/resume (to RAM) is indeed triggering the tsc switch by KVM here. This should be the first issue as the kernel itself has no problems with recovering from suspend/resume /wrt tsc. The next one is what happened to the guest running at that point. It was a SUSE 11.3 32-bit image, using kvm-clock. After resume and host-side clock switch it lost its timer ticks, likely due to some breakage of kvm-clock. And finally, I'm now in the original failure state again in which every newly started Linux guest with kvm-clock support also suffers from stuck timers. Linux kernel that lack kvm-clock run fine, e.g. on hpet clocksource. Maybe this is just another symptom of what also cause the second problem. > > This suggests two debugging techniques: I can manually switch the > clocksource, and I can also load a module which does nothing other than > mark the TSC unstable. Failing that, we can investigate PM suspend / > resume for possible issues. > > I'll try this on my Intel boxes to see what happens. Do you think kvm-kmod could contribute to this? As I said, I'm on a 34 kernel, namely SUSE's 2.6.34.4-0.1-desktop. Any feature missing in that kernel latest KVM depends for proper tsc/kvm-clock handling? If you have any concerns, I could try to run kvm.git natively later on. Jan
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