rcu_sched self-detected stall on CPU

[Paul E. McKenney]

On Fri, Apr 08, 2022 at 05:23:32PM +1000, Michael Ellerman wrote:
> "Paul E. McKenney" <paulmck at kernel.org> writes:
> > On Wed, Apr 06, 2022 at 05:31:10PM +0800, Zhouyi Zhou wrote:
> >> Hi
> >> 
> >> I can reproduce it in a ppc virtual cloud server provided by Oregon
> >> State University.  Following is what I do:
> >> 1) curl -l https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/snapshot/linux-5.18-rc1.tar.gz
> >> -o linux-5.18-rc1.tar.gz
> >> 2) tar zxf linux-5.18-rc1.tar.gz
> >> 3) cp config linux-5.18-rc1/.config
> >> 4) cd linux-5.18-rc1
> >> 5) make vmlinux -j 8
> >> 6) qemu-system-ppc64 -kernel vmlinux -nographic -vga none -no-reboot
> >> -smp 2 (QEMU 4.2.1)
> >> 7) after 12 rounds, the bug got reproduced:
> >> (http://154.223.142.244/logs/20220406/qemu.log.txt)
> >
> > Just to make sure, are you both seeing the same thing?  Last I knew,
> > Zhouyi was chasing an RCU-tasks issue that appears only in kernels
> > built with CONFIG_PROVE_RCU=y, which Miguel does not have set.  Or did
> > I miss something?
> >
> > Miguel is instead seeing an RCU CPU stall warning where RCU's grace-period
> > kthread slept for three milliseconds, but did not wake up for more than
> > 20 seconds.  This kthread would normally have awakened on CPU 1, but
> > CPU 1 looks to me to be very unhealthy, as can be seen in your console
> > output below (but maybe my idea of what is healthy for powerpc systems
> > is outdated).  Please see also the inline annotations.
> >
> > Thoughts from the PPC guys?
> 
> I haven't seen it in my testing. But using Miguel's config I can
> reproduce it seemingly on every boot.
> 
> For me it bisects to:
> 
>   35de589cb879 ("powerpc/time: improve decrementer clockevent processing")
> 
> Which seems plausible.
> 
> Reverting that on mainline makes the bug go away.

Thank you for looking into this!

> I don't see an obvious bug in the diff, but I could be wrong, or the old
> code was papering over an existing bug?
> 
> I'll try and work out what it is about Miguel's config that exposes
> this vs our defconfig, that might give us a clue.

I have recently had some RCU bugs that were due to Kconfig failing to
rule out broken .config files.  Maybe this is something similar?

							Thanx, Paul