Re: [PATCH v9 00/12] Support PPTT for ARM64

[Robin Murphy]

On 29/05/18 16:51, Geert Uytterhoeven wrote:
> Hi Will,
>
> On Tue, May 29, 2018 at 5:08 PM, Will Deacon <will.deacon@xxxxxxx> wrote:
> > On Tue, May 29, 2018 at 02:18:40PM +0100, Sudeep Holla wrote:
> > > On 29/05/18 12:56, Geert Uytterhoeven wrote:
> > > > On Tue, May 29, 2018 at 1:14 PM, Sudeep Holla <sudeep.holla@xxxxxxx> wrote:
> > > > > On 29/05/18 11:48, Geert Uytterhoeven wrote:
> > > > > > System supend still works fine on systems with big cores only:
> > > > > >
> > > > > >      R-Car H3 ES1.0 (4xCA57 (4xCA53 disabled in firmware))
> > > > > >      R-Car M3-N (2xCA57)
> > > > > >
> > > > > > Reverting this commit fixes the issue for me.
> > > > >
> > > > > I can't find anything that relates to system suspend in these patches
> > > > > unless they are messing with something during CPU hot plug-in back
> > > > > during resume.
> > > >
> > > > It's only the last patch that introduces the breakage.
> > >
> > > As specified in the commit log, it won't change any behavior for DT
> > > systems if it's non-NUMA or single node system. So I am still wondering
> > > what could trigger this regression.
> >
> > I wonder if we're somehow giving an uninitialised/invalid NUMA configuration
> > to the scheduler, although I can't see how this would happen.
> >
> > Geert -- if you enable CONFIG_DEBUG_PER_CPU_MAPS=y and apply the diff below
> > do you see anything shouting in dmesg?
>
> Thanks, but unfortunately it doesn't help.
> I added some debug code to print cpumask, but so far I don't see anything
> suspicious.

Do you have CONFIG_NUMA enabled? On a hunch I've managed to reproduce 
what looks like the same thing on a Juno board with NUMA=n; going in 
with external debug it seems to be stuck in the loop in 
init_sched_groups_capacity(), with an approximate stack trace of:


init_sched_groups_capacity()
partition_sched_domains()
cpuset_cpu_active()
sched_cpu_activate()
cpuhp_invoke_callback()
cpuhp_thread_fn()

My hunch is based on the fact that it looks like we can, under the right 
circumstances, end up with default_topology picking up cpu_online_mask 
as a sibling mask via cpu_coregroup_mask(), and given the great 
coincidence that that's going to change when hotplugging out CPUs on 
suspend, things might not react too well to that. Things also look to go 
utterly haywire once into a full-blown systemd userspace with cpuidle, 
but I haven't got a clear picture of that yet.

Robin.