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

[Nicholas Piggin]

Excerpts from Nicholas Piggin's message of October 19, 2020 11:00 am:
> Excerpts from Michal Suchánek's message of October 17, 2020 6:14 am:
>> 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.
>> 
>> Hello,
>> 
>> I tried running some KVM guests to simulate the workload and what I get
>> is guests failing to start with a rcu stall. Tried both 5.3 and 5.9
>> kernel and qemu 4.2.1 and 5.1.0
>> 
>> To start some guests I run
>> 
>> for i in $(seq 0 9) ; do /opt/qemu/bin/qemu-system-ppc64 -m 2048 -accel kvm -smp 8 -kernel /boot/vmlinux -initrd /boot/initrd -nodefaults -nographic -serial mon:telnet::444$i,server,wait & done
>> 
>> To simulate some workload I run
>> 
>> xz -zc9T0 < /dev/zero > /dev/null &
>> while true; do
>>     killall -STOP xz; sleep 1; killall -CONT xz; sleep 1;
>> done &
>> 
>> on the host and add a job that executes this to the ramdisk. However, most
>> guests never get to the point where the job is executed.
>> 
>> Any idea what might be the problem?
> 
> I would say try without pv queued spin locks (but if the same thing is 
> happening with 5.3 then it must be something else I guess). 
> 
> I'll try to test a similar setup on a POWER8 here.

Couldn't reproduce the guest hang, they seem to run fine even with 
queued spinlocks. Might have a different .config.

I might have got a lockup in the host (although different symptoms than 
the original report). I'll look into that a bit further.

Thanks,
Nick