Re: [Qemu-devel] [ceph-users] qemu-1.4.0 and onwards, linux kernel 3.2.x, ceph-RBD, heavy I/O leads to kernel_hung_tasks_timout_secs message and unresponsive qemu-process, [Bug 1207686]

[Mike Dawson <mike.dawson@cloudapt.com>]

Josh,

Logs are uploaded to cephdrop with the file name 
mikedawson-rbd-qemu-deadlock.

- At about 2013-08-05 19:46 or 47, we hit the issue, traffic went to 0
- At about 2013-08-05 19:53:51, ran a 'virsh screenshot'


Environment is:

- Ceph 0.61.7 (client is co-mingled with three OSDs)
- rbd cache = true and cache=writeback
- qemu 1.4.0 1.4.0+dfsg-1expubuntu4
- Ubuntu Raring with 3.8.0-25-generic

This issue is reproducible in my environment, and I'm willing to run any 
wip branch you need. What else can I provide to help?

Thanks,
Mike Dawson


On 8/5/2013 3:48 AM, Stefan Hajnoczi wrote:
> On Sun, Aug 04, 2013 at 03:36:52PM +0200, Oliver Francke wrote:
>> Am 02.08.2013 um 23:47 schrieb Mike Dawson <mike.dawson@cloudapt.com>:
>>> We can "un-wedge" the guest by opening a NoVNC session or running a 'virsh screenshot' command. After that, the guest resumes and runs as expected. At that point we can examine the guest. Each time we'll see:
>
> If virsh screenshot works then this confirms that QEMU itself is still
> responding.  Its main loop cannot be blocked since it was able to
> process the screendump command.
>
> This supports Josh's theory that a callback is not being invoked.  The
> virtio-blk I/O request would be left in a pending state.
>
> Now here is where the behavior varies between configurations:
>
> On a Windows guest with 1 vCPU, you may see the symptom that the guest no
> longer responds to ping.
>
> On a Linux guest with multiple vCPUs, you may see the hung task message
> from the guest kernel because other vCPUs are still making progress.
> Just the vCPU that issued the I/O request and whose task is in
> UNINTERRUPTIBLE state would really be stuck.
>
> Basically, the symptoms depend not just on how QEMU is behaving but also
> on the guest kernel and how many vCPUs you have configured.
>
> I think this can explain how both problems you are observing, Oliver and
> Mike, are a result of the same bug.  At least I hope they are :).
>
> Stefan
>