ftrace hangs waiting for rcu

[Mark Rutland]

On Fri, Jan 28, 2022 at 05:08:48PM +0100, Sven Schnelle wrote:
> Hi Mark,
> 
> Mark Rutland <mark.rutland at arm.com> writes:
> 
> > On arm64 I bisected this down to:
> >
> >   7a30871b6a27de1a ("rcu-tasks: Introduce ->percpu_enqueue_shift for dynamic queue selection")
> >
> > Which was going wrong because ilog2() rounds down, and so the shift was wrong
> > for any nr_cpus that was not a power-of-two. Paul had already fixed that in
> > rcu-next, and just sent a pull request to Linus:
> >
> >   https://lore.kernel.org/lkml/20220128143251.GA2398275@paulmck-ThinkPad-P17-Gen-1/
> >
> > With that applied, I no longer see these hangs.
> >
> > Does your s390 test machine have a non-power-of-two nr_cpus, and does that fix
> > the issue for you?
> 
> We noticed the PR from Paul and are currently testing the fix. So far
> it's looking good. The configuration where we have seen the hang is a
> bit unusual:
> 
> - 16 physical CPUs on the kvm host
> - 248 logical CPUs inside kvm

Aha! 248 is notably *NOT* a power of two, and in this case the shift would be
wrong (ilog2() would give 7, when we need a shift of 8).

So I suspect you're hitting the same issue as I was.

Thanks,
Mark.

> - debug kernel both on the host and kvm guest
> 
> So things are likely a bit slow in the kvm guest. Interesting is that
> the number of CPUs is even. But maybe RCU sees an odd number of CPUs
> and gets confused before all cpus are brought up. Have to read code/test
> to see whether that could be possible.
> 
> Thanks for investigating!
> Sven