arm64 torture test hotplug failures (offlining causes -EBUSY)

[Zhouyi Zhou]

Hi Joel

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.

[1] https://lore.kernel.org/lkml/20221127175317.GF4001@paulmck-ThinkPad-P17-Gen-1/T/

Thanks
Zhouyi
> You can browse through all the torture test artifacts here which
> contains everything (build logs, vmlinux, etc).
> 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/
>
> Thanks for your help!
>
>  - Joel