arm64 torture test hotplug failures (offlining causes -EBUSY)

[Paul E. McKenney]

On Mon, Jan 16, 2023 at 11:36:57PM -0500, Joel Fernandes wrote:
> > On Jan 16, 2023, at 11:30 PM, Paul E. McKenney <paulmck at kernel.org> wrote:
> > On Tue, Jan 17, 2023 at 12:15:07AM +0000, Joel Fernandes wrote:
> >>> On Mon, Jan 16, 2023 at 05:38:00PM -0500, Joel Fernandes wrote:
> >>> Hi Zhouyi,
> >>> 
> >>> On Mon, Jan 16, 2023 at 1:33 PM Zhouyi Zhou <zhouzhouyi at gmail.com> wrote:
> >>>> 
> >>> [..]
> >>>> On Tue, Jan 17, 2023 at 1:27 AM Joel Fernandes <joel at joelfernandes.org> wrote:
> >>>>> 
> >>>>> Hello,
> >>>>> I am seeing -EBUSY returned a lot during torture_onoff() when running
> >>>>> rcutorture on arm64. This causes hotplug failure 30% of the time. I am
> >>>>> also seeing this in 6.1-rc kernels. I believe see this only for CPU0.
> >>>>> 
> >>>>> This causes warnings in torture tests:
> >>>>> [  217.582290] rcu-torture:torture_onoff task: offline 0 failed: errno -16
> >>>>> [  221.866362] rcu-torture:torture_onoff task: offline 0 failed: errno -16
> >>>>> 
> >>>>> Full kernel log here:
> >>>>> http://box.joelfernandes.org:9080/job/rcutorture_stable_arm/job/linux-5.15.y/7/artifact/tools/testing/selftests/rcutorture/res/2023.01.15-14.51.11/TREE04/console.log
> >>>>> 
> >>>>> Any ideas on why this is happening and only for CPU 0 (presumably the
> >>>>> boot CPU)? I'd personally need these warnings to go away for my tests
> >>>>> as this causes rcutorture's tests to not cleanly pass for me. It
> >>>>> appears remove_cpu() -> device_offline() is what returns the error.
> >>>>> 
> >>>> I guess this probably because CPU 0 is the tick_do_timer_cpu in
> >>>> nohz_full mode, which prevent that cpu from
> >>>> going offline [1]. We have discussed this topic, but there is no
> >>>> agreement on how to solve it yet.
> >>> 
> >>> But I am seeing the issue in TRACE02 config which is:
> >>> CONFIG_NO_HZ_IDLE=y
> >>> # CONFIG_NO_HZ_FULL is not set
> >>> 
> >>> So that is not NO_HZ_FULL:
> >>> http://box.joelfernandes.org:9080/job/rcutorture_stable_arm/job/linux-5.15.y/7/artifact/tools/testing/selftests/rcutorture/res/2023.01.15-14.51.11/TRACE02/console.log.diags/
> >>> However, I can't seem to find the full kernel logs for that.
> >>> 
> >>> Also, other than the TRACE02 fail, I only see the issue with configs
> >>> with CONFIG_NO_HZ_FULL=y
> >>> 
> >>> Can you try TRACE02 specifically, and see if you can reproduce the
> >>> same issue on your setup? Meanwhile, I'll try to trace what is
> >>> returning the -EBUSY.
> >> 
> >> How about something simple like the following? (untested)
> >> 
> >> ---8<-----------------------
> >> 
> >> diff --git a/kernel/torture.c b/kernel/torture.c
> >> index bc8fb361efc0..cd64110694c0 100644
> >> --- a/kernel/torture.c
> >> +++ b/kernel/torture.c
> >> @@ -220,6 +220,9 @@ bool torture_offline(int cpu, long *n_offl_attempts, long *n_offl_successes,
> >>            // PCI probe frequently disables hotplug during boot.
> >>            (*n_offl_attempts)--;
> >>            s = " (-EBUSY forgiven during boot)";
> >> +        } else if (tick_nohz_full_running && ret == -EBUSY) {
> >> +            (*n_offl_attempts)--;
> >> +            s = " (-EBUSY forgiven if nohz_full is running)";
> > 
> > But this should be forgiven for the timekeeping CPU, not everyone,
> > correct?
> > 
> > Yes, I know that CPU-hotplug operations can fail, but in my testing
> > they almost never do.  This means that a new failure might well be a
> > real bug somewhere that needs attention.
> 
> Sure. We may need to expose some API to reveal that. 
> 
> It appeared though that Thomas in the other thread related to patch
> from Zhouyi, was suggesting that rcutorture tolerate hotplug failure
> though, because they are not abnormal, right?

Based on my rcutorture testing experience on x86, they are not at all
normal.  The only time I have seen rcutorture CPU-hotplug failures has
been due to some bug that needed fixing.

Is there a plan to make CPU hotplug failures more frequent?

							Thanx, Paul