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

[Nicholas Piggin]

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?

Thanks,
Nick