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

[Nicholas Piggin]

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. Do the messages always come in pairs of CPUs?

I'm not sure where to start with reproducing, I'll have to try. How many
vCPUs in the guests? Do you have several guests running at once?

Thanks,
Nick