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]

[Josh Durgin]

On 08/08/2013 05:40 AM, Oliver Francke wrote:
> Hi Josh,
>
> I have a session logged with:
>
>      debug_ms=1:debug_rbd=20:debug_objectcacher=30
>
> as you requested from Mike, even if I think, we do have another story
> here, anyway.
>
> Host-kernel is: 3.10.0-rc7, qemu-client 1.6.0-rc2, client-kernel is
> 3.2.0-51-amd...
>
> Do you want me to open a ticket for that stuff? I have about 5MB
> compressed logfile waiting for you ;)

Yes, that'd be great. If you could include the time when you saw the 
guest hang that'd be ideal. I'm not sure if this is one or two bugs,
but it seems likely it's a bug in rbd and not qemu.

Thanks!
Josh

> Thnx in advance,
>
> Oliver.
>
> On 08/05/2013 09:48 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 <address@hidden>:
> > > > 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