ftrace hangs waiting for rcu

[Paul E. McKenney]

On Fri, Jan 28, 2022 at 08:15:47AM -0800, Paul E. McKenney wrote:
> On Fri, Jan 28, 2022 at 04:11:57PM +0000, Mark Rutland wrote:
> > 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.
> 
> And apparently no one runs -next on systems having a non-power-of-two
> number of CPUs.  ;-)

And the fix is now in mainline.

							Thanx, Paul

> > 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