Re: 2.6.35-rc1 regression with pvclock and smp guests

Andre Przywara
2010-08-02T10:17:02Z

Zachary Amsden wrote:
> On 07/28/2010 02:25 AM, Andre Przywara wrote:
>> Andre Przywara wrote:
>>> Andre Przywara wrote:
>>>> 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.
>>> Did some more investigations, some observations:
>>> - The cmpxchg does not seem to be a problem, I didn't see the loop
>>> iterated more than once.
>>> - Turning off printk-timestamps makes the bug go away. But I guess it
>>> is just hiding or deferring it, and it's no real workaround anyway.
>>> - I instrumented the "if (ret < last) return last;" statement, when
>>> the kernel hangs I get only printks from there, although it has hit
>>> before:
>>> ----------
>>> [ 0.820000] Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled
>>> [ 0.820000] returning last instead (cnt=19001)
>>> [ 0.820000] returning last instead (cnt=20001)
>>> The last line repeats forever with the same timestamp, the counter
>>> (counting the number of "return last;") increments about 3500
>>> times/second.
>>>
>>> I will see if I find something more...
>> Added some more instrumentation, seems like the values read from the
>> pvclock is bogus *sometimes*:
>> returning last instead (2778021535795841, cnt=1, diff=1389078312510470)
>> This is from the first time the if-statement triggers. So I guess the
>> value read is ridiculously far in the future (multiple days), so next
>> calls to clocksource_read() will always return this bogus last value.
>> This means that the clock does not make progress (for several days)
>> and thus timing loops will never come to an end. I also instrumented
>> the serial driver, the last thing I saw was autoconfig_irq, where
>> obviously udelay() is called.
>>
>> Does that ring a bell with someone?
>>
>> I will now concentrate on the pvclock readout/HV write part to see
>> which of the values used here are wrong.
>
> Have you gotten any further results on this?
Somehow. I think my latest findings were more or less ghost bugs: since
prinkts contain a timestamp they interfere with the actual code. The
large gap I described above was only to be seen with these printks, it
is more or less double the real value (which is my host's uptime).
Sadly I cannot use debugfs to avoid the printks, since the kernel halts
and I don't get to userland.

On another try I managed to bisect the failure also in qemu-kvm. The bug
triggers only with "ebc4f45 turn off kvmclock when resetting cpu"
applied (_additionally_ to the kernel patch in question).
When I comment out the call to kvm_reset_msrs() in the master branch,
this also lets the bug vanish.
>
> I think the most likely explanation is that your host CPU has TSC out of
> sync, and somehow this leaks over to pvclock. Am I correct that it
> happens even with one guest VCPU? What if you disable secondary host CPUs?
I tried several ways to pin VCPUs to different host CPUs (cores and
sockets): both VCPUs on one core, both vCPUs on different cores on the
same socket and both vCPUs to different sockets/nodes. That all did not
make any difference, the kernel halted in either case.
I also tried booting the host with maxcpus=1, the error was still the
same: -smp 1 works, -smp 2 halts.
Btw.: the host uses clocksource acpi_pm. Also I noticed that sometimes
the guests gets very slow after having switched the clocksource to
kvmclock, it then eventually halts at the mentioned line.

Regards,
Andre.

--
Andre Przywara
AMD-OSRC (Dresden)
Tel: x29712