Re: [ANNOUNCE] Xen 4.15 - call for notification/status of significant bugs

[Tamas K Lengyel <tamas.k.lengyel@gmail.com>]

On Thu, Feb 4, 2021 at 9:21 AM Dario Faggioli <dfaggioli@suse.com> wrote:
>
> On Thu, 2021-02-04 at 12:12 +0000, Ian Jackson wrote:
> > B. "scheduler broken" bugs.
> >
> > Information from
> >   Andrew Cooper <andrew.cooper3@citrix.com>
> >   Dario Faggioli <dfaggioli@suse.com>
> >
> > Quoting Andrew Cooper
> > > We've had 4 or 5 reports of Xen not working, and very little
> > > investigation on whats going on.  Suspicion is that there might be
> > > two bugs, one with smt=0 on recent AMD hardware, and one more
> > > general "some workloads cause negative credit" and might or might
> > > not be specific to credit2 (debugging feedback differs - also might
> > > be 3 underlying issue).
> >
> > I reviewed a thread about this and it is not clear to me where we are
> > with this.
> >
> Ok, let me try to summarize the current status.
>
> - BUG: credit=sched2 machine hang when using DRAKVUF
>
>   https://lists.xen.org/archives/html/xen-devel/2020-05/msg01985.html
>   https://lists.xenproject.org/archives/html/xen-devel/2020-10/msg01561.html
>   https://bugzilla.opensuse.org/show_bug.cgi?id=1179246
>
>   99% sure that it's a Credit2 scheduler issue.
>   I'm actively working on it.
>   "Seems a tricky one; I'm still in the analysis phase"
>
>   Manifests only with certain combination of hardware and workload.
>   I'm not reproducing, but there are multiple reports of it (see
>   above). I'm investigating and trying to come up at least with
>   debug patches that one of the reporter should be able and willing to
>   test.
>
> - Null scheduler and vwfi native problem
>
>   https://lists.xenproject.org/archives/html/xen-devel/2021-01/msg01634.html
>
>   RCU issues, but manifests due to scheduler behavior (especially
>   NULL scheduler, especially on ARM).
>   I'm actively working on it.
>
>   Patches that should solve the issue for ARM posted already. They
>   will need to be slightly adjusted to cover x86 as well. Waiting a
>   couple days more for a confirmation from the reporter that the
>   patches do help, at least on ARM.
>

I've run into null-scheduler causing CPU lockups as well on x86.
Required physical machine reboot. Seems to be triggered with domain
destruction when destroying fork vms. Happens only intermittently.

Tamas