Re: [KVM timekeeping 10/35] Fix deep C-state TSC desynchronization

[Zachary Amsden]

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.

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.
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