KVM on POWER8 host lock up since 10d91611f426 ("powerpc/64s: Reimplement book3s idle code in C")

[Michal Suchánek]

On Mon, Sep 07, 2020 at 11:13:47PM +1000, Nicholas Piggin wrote:
> Excerpts from Michael Ellerman's message of August 31, 2020 8:50 pm:
> > Michal Suchánek <msuchanek at suse.de> writes:
> >> On Mon, Aug 31, 2020 at 11:14:18AM +1000, Nicholas Piggin wrote:
> >>> Excerpts from Michal Suchánek's message of August 31, 2020 6:11 am:
> >>> > Hello,
> >>> > 
> >>> > on POWER8 KVM hosts lock up since commit 10d91611f426 ("powerpc/64s:
> >>> > Reimplement book3s idle code in C").
> >>> > 
> >>> > The symptom is host locking up completely after some hours of KVM
> >>> > workload with messages like
> >>> > 
> >>> > 2020-08-30T10:51:31+00:00 obs-power8-01 kernel: KVM: couldn't grab cpu 47
> >>> > 2020-08-30T10:51:31+00:00 obs-power8-01 kernel: KVM: couldn't grab cpu 71
> >>> > 2020-08-30T10:51:31+00:00 obs-power8-01 kernel: KVM: couldn't grab cpu 47
> >>> > 2020-08-30T10:51:31+00:00 obs-power8-01 kernel: KVM: couldn't grab cpu 71
> >>> > 2020-08-30T10:51:31+00:00 obs-power8-01 kernel: KVM: couldn't grab cpu 47
> >>> > 
> >>> > printed before the host locks up.
> >>> > 
> >>> > The machines run sandboxed builds which is a mixed workload resulting in
> >>> > IO/single core/mutiple core load over time and there are periods of no
> >>> > activity and no VMS runnig as well. The VMs are shortlived so VM
> >>> > setup/terdown is somewhat excercised as well.
> >>> > 
> >>> > POWER9 with the new guest entry fast path does not seem to be affected.
> >>> > 
> >>> > Reverted the patch and the followup idle fixes on top of 5.2.14 and
> >>> > re-applied commit a3f3072db6ca ("powerpc/powernv/idle: Restore IAMR
> >>> > after idle") which gives same idle code as 5.1.16 and the kernel seems
> >>> > stable.
> >>> > 
> >>> > Config is attached.
> >>> > 
> >>> > I cannot easily revert this commit, especially if I want to use the same
> >>> > kernel on POWER8 and POWER9 - many of the POWER9 fixes are applicable
> >>> > only to the new idle code.
> >>> > 
> >>> > Any idea what can be the problem?
> >>> 
> >>> So hwthread_state is never getting back to to HWTHREAD_IN_IDLE on
> >>> those threads. I wonder what they are doing. POWER8 doesn't have a good
> >>> NMI IPI and I don't know if it supports pdbg dumping registers from the
> >>> BMC unfortunately.
> >>
> >> It may be possible to set up fadump with a later kernel version that
> >> supports it on powernv and dump the whole kernel.
> > 
> > Your firmware won't support it AFAIK.
> > 
> > You could try kdump, but if we have CPUs stuck in KVM then there's a
> > good chance it won't work :/
> 
> I haven't had any luck yet reproducing this still. Testing with sub 
> cores of various different combinations, etc. I'll keep trying though.
> 
> I don't know if there's much we can add to debug it. Can we run pdbg
> on the BMCs on these things?

I suppose it depends on the machine type?

Thanks

Michal