Re: [PATCH 0/9] sched: Migrate disable support

[Valentin Schneider <valentin.schneider@arm.com>]

On 25/09/20 12:58, Dietmar Eggemann wrote:
> On 25/09/2020 12:10, Peter Zijlstra wrote:
>> On Fri, Sep 25, 2020 at 11:12:09AM +0200, Dietmar Eggemann wrote:
>>
>>> I get this when running 6 (periodic) RT50 tasks with CPU hp stress on my
>>> 6 CPU JUNO board (!CONFIG_PREEMPT_RT).
>>>
>>> [   55.490263] ------------[ cut here ]------------
>>> [   55.505261] Modules linked in:
>>> [   55.508322] CPU: 3 PID: 24 Comm: migration/3 Not tainted
>>> 5.9.0-rc1-00132-gc096e6406c50-dirty #90
>>> [   55.517119] Hardware name: ARM Juno development board (r0) (DT)
>>> [   55.523058] Stopper: multi_cpu_stop+0x0/0x170 <- 0x0
>>> [   55.528029] pstate: 20000085 (nzCv daIf -PAN -UAO BTYPE=--)
>>> [   55.533612] pc : sched_cpu_dying+0x124/0x130
>>> [   55.537887] lr : sched_cpu_dying+0xd8/0x130
>>> [   55.542071] sp : ffff800011f0bca0
>>> [   55.545385] x29: ffff800011f0bca0 x28: 0000000000000002
>>> [   55.550703] x27: 0000000000000000 x26: 0000000000000060
>>> [   55.556022] x25: 0000000000000000 x24: 0000000000000001
>>> [   55.561340] x23: 0000000000000000 x22: 0000000000000003
>>> [   55.566659] x21: 0000000000000080 x20: 0000000000000003
>>> [   55.571977] x19: ffff00097ef9e1c0 x18: 0000000000000010
>>> [   55.577295] x17: 0000000000000000 x16: 0000000000000000
>>> [   55.582613] x15: 0000000000000000 x14: 000000000000015c
>>> [   55.587932] x13: 0000000000000000 x12: 00000000000006f1
>>> [   55.593250] x11: 0000000000000080 x10: 0000000000000000
>>> [   55.598567] x9 : 0000000000000003 x8 : ffff0009743f5900
>>> [   55.603886] x7 : 0000000000000003 x6 : 0000000000000000
>>> [   55.609204] x5 : 0000000000000001 x4 : 0000000000000002
>>> [   55.614521] x3 : 0000000000000000 x2 : 0000000000000013
>>> [   55.619839] x1 : 0000000000000008 x0 : 0000000000000003
>>> [   55.625158] Call trace:
>>> [   55.627607]  sched_cpu_dying+0x124/0x130
>>> [   55.631535]  cpuhp_invoke_callback+0x88/0x210
>>> [   55.635897]  take_cpu_down+0x7c/0xd8
>>> [   55.639475]  multi_cpu_stop+0xac/0x170
>>> [   55.643227]  cpu_stopper_thread+0x98/0x130
>>> [   55.647327]  smpboot_thread_fn+0x1c4/0x280
>>> [   55.651427]  kthread+0x140/0x160
>>> [   55.654658]  ret_from_fork+0x10/0x34
>>> [   55.658239] Code: f000e1c1 913fc021 1400034a 17ffffde (d4210000)
>>> [   55.664342] ---[ end trace c5b8988b7b701e56 ]---
>>> [   55.668963] note: migration/3[24] exited with preempt_count 3
>>>
>>> 7309 int sched_cpu_dying(unsigned int cpu)
>>>     ...
>>>     BUG_ON(rq->nr_running != 1 || rq_has_pinned_tasks(rq));
>>>     ...
>>>
>>> rq->nr_running is always 2 here in this cases.
>>>
>>> balance_hotplug_wait and sched_cpu_wait_empty run in cpuhp/X (CFS)
>>> whereas sched_cpu_dying in migration/X ?
>>
>> takedown_cpu() has:
>>
>>   kthread_park(per_cpu_ptr(&cpuhp_state, cpu)->thread);
>>
>> before calling:
>>
>>   err = stop_machine_cpuslocked(take_cpu_down, NULL, cpumask_of(cpu));
>>
>> So when we get to sched_cpu_dying(), the only task that _should_ still
>> be there is migration/X.
>>
>> Do you have any idea what thread, other than migration/X, is still
>> active on that CPU? per sched_cpu_wait_empty() we should've pushed out
>> all userspace tasks, and the cpu hotplug machinery should've put all the
>> per-cpu kthreads to sleep at this point.
>
> With Valentin's print_rq() inspired test snippet I always see one of the
> RT user tasks as the second guy? BTW, it has to be RT tasks, never
> triggered with CFS tasks.
>
> [   57.849268] CPU2 nr_running=2
> [   57.852241]  p=migration/2
> [   57.854967]  p=task0-0

I can also trigger the BUG_ON() using the built-in locktorture module
(+enabling hotplug torture), and it happens very early on. I can't trigger
it under qemu sadly :/ Also, in my case it's always a kworker:

[    0.830462] CPU3 nr_running=2
[    0.833443]  p=migration/3
[    0.836150]  p=kworker/3:0

I'm looking into what workqueue.c is doing about hotplug...