Re: dozens of qemu/kvm VMs getting into stuck states since kernel ~5.13

[Chris Murphy]

On Tue, Dec 7, 2021 at 5:25 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Tue, Dec 07, 2021, Chris Murphy wrote:
> > cc: qemu-devel
> >
> > Hi,
> >
> > I'm trying to help progress a very troublesome and so far elusive bug
> > we're seeing in Fedora infrastructure. When running dozens of qemu-kvm
> > VMs simultaneously, eventually they become unresponsive, as well as
> > new processes as we try to extract information from the host about
> > what's gone wrong.
>
> Have you tried bisecting?  IIUC, the issues showed up between v5.11 and 
> v5.12.12,
> bisecting should be relatively straightforward.

We haven't tried bisecting. Due to limited access since it's a
production machine, and limited resources for those who have that
access, I think the chance of bisecting is low, but I've asked. We
could do something of a faux-bisect by running already built kernels
in Fedora infrastructure. We could start by running x.y.0 kernels to
see when it first appeared, then once hitting the problem, start
testing rc1, rc2, ... in that series. We also have approximately daily
git builds in between those rc's. That might be enough to deduce a
culprit, but I'm not sure. At the least this would get us a ~1-3 day
window within two rc's for bisecting.

>
> > Systems (Fedora openQA worker hosts) on kernel 5.12.12+ wind up in a
> > state where forking does not work correctly, breaking most things
> > https://bugzilla.redhat.com/show_bug.cgi?id=2009585
> >
> > In subsequent testing, we used newer kernels with lockdep and other
> > debug stuff enabled, and managed to capture a hung task with a bunch
> > of locks listed, including kvm and qemu processes. But I can't parse
> > it.
> >
> > 5.15-rc7
> > https://bugzilla-attachments.redhat.com/attachment.cgi?id=1840941
> > 5.15+
> > https://bugzilla-attachments.redhat.com/attachment.cgi?id=1840939
> >
> > If anyone can take a glance at those kernel messages, and/or give
> > hints how we can extract more information for debugging, it'd be
> > appreciated. Maybe all of that is normal and the actual problem isn't
> > in any of these traces.
>
> All the instances of
>
>   (&vcpu->mutex){+.+.}-{3:3}, at: kvm_vcpu_ioctl+0x77/0x720 [kvm]
>
> are uninteresting and expected, that's just each vCPU task taking its 
> associated
> vcpu->mutex, likely for KVM_RUN.
>
> At a glance, the XFS stuff looks far more interesting/suspect.

Thanks for the reply.

-- 
Chris Murphy