Re: 2.6.35-rc1 regression with pvclock and smp guests

[Avi Kivity <avi@redhat.com>]

  On 07/27/2010 03:21 PM, Andre Przywara wrote:
> Avi Kivity wrote:
>>   On 07/27/2010 02:49 PM, Andre Przywara wrote:
>>>> What is the guest executing when it hangs?
>>> Both VCPUs are halted, the monitor and System.map tell me it's in 
>>> native_safe_halt().
>>> The code sequence confirms this, it is an intentional sti;hlt 
>>> condition.
>>> Using -smp 16 also shows that all 16 VCPUs are stuck.
>>>
>>
>> Well, strange.  The intent of that patch was to make the clock never 
>> go backwards.  Perhaps the change made it go forwards by a large 
>> amount, and the guest is not hung, just waiting for some timer that 
>> is far in the future.
>>
>> Can you do something like
>>
>> -      if (ret < last)
>> +      if (ret < last) {
>> +            static u64 max_delta;
>> +            if (last - ret > max_delta) {
>> +                  max_delta = last - ret;
>> +                  printk("advancing kvmclock by: %llx\n", max_delta);
>> +            }
>>               return last;
>> +      }
>>
>> to see if this is happening?
> No change, it still hangs. I also don't see the printk.
> The output with smp=1 is like this:
> [    1.186549] ACPI: Power Button [PWRF]
> [    1.189204] XENFS: not registering filesystem on non-xen platform
> [    1.195001] Non-volatile memory driver v1.3
> [    1.196358] Linux agpgart interface v0.103
> [    1.197687] [drm] Initialized drm 1.1.0 20060810
> [    1.198926] [drm:i915_init] *ERROR* drm/i915 can't work without 
> intel_agp module!
> [    1.201213] Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled
> ÿ[    1.460714] serial8250: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
> [    1.463243] 00:06: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
> [    1.467153] brd: module loaded
> [    1.469245] loop: module loaded
> With smp=2 the output stops just before the strange "y" character (I 
> guess it's ASCII 255), which I assume is an artifact of the serial 
> console.
> As you can see at the timestamps, it takes some time between the last 
> shown line (1.201213) and the first missing one (1.460714).

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.

-- 
error compiling committee.c: too many arguments to function