Re: 2.6.35-rc1 regression with pvclock and smp guests

[Andre Przywara <andre.przywara@amd.com>]

Avi Kivity wrote:
>   On 07/27/2010 04:48 PM, Andre Przywara wrote:
>>> Wierd.  Maybe the clock goes crazy.
>>>
>>> Let's see if it jumps forward alot:
>>>
>>>          } while (unlikely(last != ret));
>>> +
>>> +       {
>>> +            static u64 last_report;
>>> +            if (ret > last_report + 10000) {
>>> +                    last_report = ret;
>>> +                    printk("kvmclock: %llx\n", ret);
>>> +            }
>>> +
>>> +       }
>>>
>>>          return ret;
>>>   }
>>>
>>> Worth updating the 'return last' to update ret and goto the new code, 
>>> so we don't miss that path.
>> Did that. There is _a lot_ of output (about 350 lines per second via 
>> the 115k serial console), both with smp=1 and smp=2.
>> The majority is differing about 2,000,000 (ticks?), but a handful of 
>> them are in the range of 20 million. 
> 
> nanoseconds.  So 2-20ms.  Consistent with 350 lines/sec.
> 
>> No difference between smp=2 and smp=1.
>> I also get some "BUG: recent printk recursion!" and I don't see any 
>> kernel boot progress beyond outputting the BogoMIPS value.
> 
> Right, printk() wants the time too.
> 
>> BTW: I found two message from your earlier debug statement:
>> [    0.000000] kvm-clock: cpu 0, msr 0:1ac0401, boot clock
>> [    0.000000] kvm-clock: cpu 0, msr 0:1e15401, primary cpu clock
> 
> Those are from kvmclock initialization, not from the older patch.
> 
> I'm completely confused, everything seems to be in order.
> 
> Let's see.  if you s/return last/return ret/ in the original, does this 
> help things along?  this makes pvclock drop the computation and should 
> be exactly the same as before the patch.
Yes, this works, both smp version boot. I see a short very short break 
after the line in question, but then it proceeds well.
Thanks for your help, now I got a much better insight into the issue. I 
will see if I can find something more.

Regards,
Andre.



-- 
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 448-3567-12