On Tue, Jan 17, 2023 at 08:02:24PM +0000, Joel Fernandes wrote: > On Tue, Jan 17, 2023 at 4:54 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > > > 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@xxxxxxxxxx> 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@xxxxxxxxx> wrote: > > > >>>> > > > >>> [..] > > > >>>> On Tue, Jan 17, 2023 at 1:27 AM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> 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. > > I see, ok. I need to debug what is returning -EBUSY for !NO_HZ_FULL on > arm64, I will report back once I do. > > Meanwhile, Marc I am wondering if you are able to reproduce the issue > on your side on TRACE02 config, like I am? > > Here is the TRACE02 config fragment: > 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/ConfigFragment/*view*/ > > Here are instructions on how to run it (torture test parameters etc) > if you are loading the module yourself: > 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/bare-metal/*view*/ I am assuming that this is directed to someone having easy access to ARM hardware. > > 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(). And topology_init() sets this based on platform_can_hotplug_cpu(cpu). And this function sets CPU 0 as !cpu_is_hotpluggable() unless the architecture specifies a .cpu_can_disable() function. So architectures that don't want specific CPUs to be hotpluggable can and should so specify. Thanx, Paul > Thanks, > > - Joel > [1] https://lkml.org/lkml/2022/11/27/182
