Re: WARNING at kernel/sched/core.c:2013 migration_cpu_stop+0x2e3/0x330

[Oleksandr Natalenko <oleksandr@natalenko.name>]

Hi.

On 16.11.2020 11:00, Valentin Schneider wrote:
> On 15/11/20 22:32, Oleksandr Natalenko wrote:
>> I'm running v5.10-rc3-rt7 for some time, and I came across this splat 
>> in
>> dmesg:
>> 
>> ```
>> [118769.951010] ------------[ cut here ]------------
>> [118769.951013] WARNING: CPU: 19 PID: 146 at kernel/sched/core.c:2013
> 
> Err, I didn't pick up on this back then, but isn't that check bogus? If 
> the
> task is enqueued elsewhere, it's valid for it not to be affined
> 'here'. Also that is_migration_disabled() check within is_cpu_allowed()
> makes me think this isn't the best thing to call on a remote task.
> 
> ---
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 1218f3ce1713..47d5b677585f 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2010,7 +2010,7 @@ static int migration_cpu_stop(void *data)
>  		 * valid again. Nothing to do.
>  		 */
>  		if (!pending) {
> -			WARN_ON_ONCE(!is_cpu_allowed(p, cpu_of(rq)));
> +			WARN_ON_ONCE(!cpumask_test_cpu(task_cpu(p), p->cpus_ptr));
>  			goto out;
>  		}

Not sure whether the check is legitimate, but FWIW I've managed to put a 
test task [1] (it spawns a lot of threads and applies affinity) into a 
permanent unkillable D state here:

```
[<0>] affine_move_task+0x2d3/0x620
[<0>] __set_cpus_allowed_ptr+0x164/0x210
[<0>] sched_setaffinity+0x21a/0x300
[<0>] __x64_sys_sched_setaffinity+0x8c/0xc0
[<0>] do_syscall_64+0x33/0x40
[<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
```

I think this corresponds to something around here:

```
$ scripts/faddr2line kernel/sched/core.o affine_move_task+0x2d3
affine_move_task+0x2d3/0x620:
arch_atomic_fetch_sub at 
/home/pf/linux-pf-edge/src/linux-5.10/./arch/x86/include/asm/atomic.h:190
(inlined by) atomic_fetch_sub_release at 
/home/pf/linux-pf-edge/src/linux-5.10/./include/asm-generic/atomic-instrumented.h:221
(inlined by) __refcount_sub_and_test at 
/home/pf/linux-pf-edge/src/linux-5.10/./include/linux/refcount.h:272
(inlined by) __refcount_dec_and_test at 
/home/pf/linux-pf-edge/src/linux-5.10/./include/linux/refcount.h:315
(inlined by) refcount_dec_and_test at 
/home/pf/linux-pf-edge/src/linux-5.10/./include/linux/refcount.h:333
(inlined by) affine_move_task at 
/home/pf/linux-pf-edge/src/linux-5.10/kernel/sched/core.c:2334
```

or:

```
2332     wait_for_completion(&pending->done);
2333
2334     if (refcount_dec_and_test(&pending->refs))
2335         wake_up_var(&pending->refs);
```

I'm not positive about this being directly related to the original 
report, but I think it is still worth mentioning.

Thanks.

[1] https://gitlab.com/post-factum/burn_scheduler

-- 
   Oleksandr Natalenko (post-factum)