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

[Michael Ellerman]

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 :/

cheers