arm64 torture test hotplug failures (offlining causes -EBUSY)

[Paul E. McKenney]

On Wed, Jan 18, 2023 at 10:39:28PM +0000, Joel Fernandes wrote:
> On Wed, Jan 18, 2023 at 10:37 PM Joel Fernandes <joel at joelfernandes.org> wrote:
> >
> > On Tue, Jan 17, 2023 at 08:00:58PM -0800, Paul E. McKenney wrote:
> > [...]
> > > > > > > Is there a plan to make CPU hotplug failures more frequent?
> > > > > >
> > > > > > I am not aware of such a plan but I was going by "There are quite some
> > > > > > reasons why a CPU-hotplug or a hot-unplug operation can fail, which is
> > > > > > not a fatal problem, really." in [1].
> > > > > >
> > > > > > What about an rcutorture to skip hotplug for a certain cpu id,
> > > > > > rcutorture.skip_hotplug_cpus="0". Can be a last resort. But we/I
> > > > > > should debug this issue more before getting to that.
> > > > >
> > > > > Yes, in fact there already are some checks along those lines, for example,
> > > > > the torture_offline() function's check of cpu_is_hotpluggable().  So for
> > > > > example, as I understand it, a CONFIG_NO_HZ_FULL=y system should mark
> > > > > the housekeeping CPU as !cpu_is_hotpluggable().
> > > >
> > > > I don't think CONFIG_NO_HZ_FULL does any such marking (at least I am
> > > > not seeing it). Even on x86, if you enable
> > > > CONFIG_BOOTPARAM_HOTPLUG_CPU0=y , and CONFIG_NO_HZ_FULL=y, and run
> > > > rcutorture with boot args:
> > > >
> > > > nohz_full=0-3 rcutorture.onoff_interval=100 rcutorture.onoff_holdoff=2
> > > > rcutorture.shutdown_secs=30
> > > >
> > > > You will see this in the kernel logs:
> > > > [    2.816022] rcu-torture:torture_onoff task: offline 0 failed: errno -16
> > > > [    2.975913] rcu-torture:torture_onoff task: offline 0 failed: errno -16
> > > >
> > > > So RCU torture test clearly thought the CPUs were hot-pluggable, when
> > > > they was chance for them to return -EBUSY (due to housekeeping and
> > > > what not). So this issue seems to be architecture independent, in that
> > > > sense.
> > > >
> > > > So the 2 ways forward I see are:
> > > > - Make the torture test aware of which CPUs are 'house keeping'
> > > > - Make it possible to turn off CPU0 hotplugging on ARM64 by default
> > > > (via CONFIG or boot option).
> > > >
> > > > Another option could be, forgive -EBUSY on CPU0 for
> > > > CONFIG_NO_HZ_FULL=y.  Is it possible to assign a non-0 CPU id as a
> > > > housekeeping CPU?
> > >
> > > I would be happier to forgive failure to offline housekeeping CPUs than
> > > blanket forgiveness of CPU 0.  Especially given that I recently got
> > > burned by a non-zero boot cpu.  ;-)
> > >
> > > But wouldn't it be even better for cpu_is_hotpluggable() to know the
> > > NO_HZ_FULL rules of the road?
> >
> > That's a great idea. I found a way to do that without having to do the
> > EXPORT_SYMBOL (like in Zhouyi's patch).
> >
> > Would the following be acceptable (only build-tested)?
> >
> > I can run more tests and submit a patch:
> >
> > diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
> > index 55405ebf23ab..f73bc520b70e 100644
> > --- a/drivers/base/cpu.c
> > +++ b/drivers/base/cpu.c
> > @@ -487,7 +487,8 @@ static const struct attribute_group *cpu_root_attr_groups[] = {
> >  bool cpu_is_hotpluggable(unsigned int cpu)
> >  {
> >         struct device *dev = get_cpu_device(cpu);
> > -       return dev && container_of(dev, struct cpu, dev)->hotpluggable;
> > +       return dev && container_of(dev, struct cpu, dev)->hotpluggable
> > +               && !tick_nohz_cpu_hotpluggable(cpu);
> 
> Oops, I should lose that "!" , but otherwise should be ok.

Looks plausible to me, but I must defer to Frederic and the various
architecture maintainers.

							Thanx, Paul