Re: [Xen-devel] State of GPLPV tests - 28.11.11

[Andreas Kinzler <ml-xen-devel@xxxxxx>]

On 29.11.2011 00:16, James Harper wrote:
> > I am still running tests 7 days a week on two test systems. Results are quite
> > discouraging though. After experiencing crash after crash I wanted to test if
> > the configuration I called "stable" (Xen 4.0.1, GPLPV 0.11.0.213, dom0 kernel
> > 2.6.32.18-pvops0-ak3) was stable indeed. But even that config crashed when
> > running my torture test. It is stable on our production systems - running
> > other workloads of course.
> What crash are you getting these days? Is it the same one as you used to
> get?
Yes, still exactly the same crashes.

Good good news: I think I have found the bug. Since I am not really a 
Xen or Windows kernel developer it cannot say for sure but here is what 
I found:
When domU hang I ran xentop and found out that the number of vbd read 
requests was an number like 0x7FFFzzzz in hex which lead me to a thesis: 
GPLPV crashes as soon as the number of disk requests reaches 2^32. On my 
hardware with 5000 IIOPs/sec this is reached in
2^32 / 5000 IIOPs / 3600 sec-per-hour / 24 hours-per-day = 9.94 days
And there we go: there are the 9-10 days I was always seeing.

I studied the source code of blkback/blktap/aio and found nothing. But 
in GPLPV and its use of the ring macros I found suspicious code in every 
version of GPLPV I ever used
  while (more_to_do)
  {
    rp = xvdd->ring.sring->rsp_prod;
    KeMemoryBarrier();
    for (i = xvdd->ring.rsp_cons; i < rp; i++)
    {
      rep = XenVbd_GetResponse(xvdd, i);

If now rp is 10 for example and xvdd->ring.rsp_cons is 0xFFFFFFF7 then 
the for loop is skipped, responses are not delivered and we see the hang.
Regards Andreas


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