Re: [PATCH v3] sched/rt: fix bad task migration for rt tasks

[Schspa Shi]

Steven Rostedt <rostedt@xxxxxxxxxxx> writes:

> On Fri,  8 Jul 2022 00:50:14 +0800
> Schspa Shi <schspa@xxxxxxxxx> wrote:
>
> > Please refer to the following scenarios.
>
> I'm not sure this is what is happening. Do you have a trace to 
> back this up?

I don't have a trace. This is inferred from the exception log.

> >            CPU0                                  CPU1
> > ------------------------------------------------------------------
> > push_rt_task
> >   check is_migration_disabled(next_task)
> >                                         task not running and
> >                                         migration_disabled == 0
> >   find_lock_lowest_rq(next_task, rq);
> >     _double_lock_balance(this_rq, busiest);
> >       raw_spin_rq_unlock(this_rq);
> >       double_rq_lock(this_rq, busiest);
> >         <<wait for busiest rq>>
> >                                             <wakeup>
>
> Here's the problem I have. next_task is queued on CPU0, 
> (otherwise CPU0
> would not be pushing it). As CPU0 is currently running 
> push_rt_task, how
> did next_task start running to set its migrate_disable flag?

THe next_task wasn't queued on CPU0, it's queued on CPU1 in this
scenarios.

And it's because when task wakup, the rq argument is not the
current running CPU rq, it's next_task's rq
(i.e. CPU1's rq in this sample scenarios).

And you can check this with the Call trace from the crash log.

   [123671.996969] Call trace:
   [123671.996975]  set_task_cpu+0x8c/0x108
   [123671.996984]  push_rt_task.part.0+0x144/0x184
   [123671.996995]  push_rt_tasks+0x28/0x3c
   [123671.997002]  task_woken_rt+0x58/0x68
   [123671.997009]  ttwu_do_wakeup+0x5c/0xd0
   [123671.997019]  ttwu_do_activate+0xc0/0xd4
   [123671.997028]  try_to_wake_up+0x244/0x288
   [123671.997036]  wake_up_process+0x18/0x24
   [123671.997045]  __irq_wake_thread+0x64/0x80
   [123671.997056]  __handle_irq_event_percpu+0x110/0x124

Function ttwu_do_wakeup will lock the task's rq, not current 
running
cpu rq.

> Even if it was woken up on another CPU and ran there, by setting
> migrate_disable, it would not be put back to CPU0, because its
> migrate_disable flag is set (if it is, then there's the bug).

It no needs to put it back to CPU0 for this issue, it's still on 
CPU1.

> After releasing the rq lock and retaking it, we check that the 
> next_task is
> still the next task on CPU0 to push.
>
>
> >                                         task become running
> >                                         migrate_disable();
> >                                           <context out>
> >   deactivate_task(rq, next_task, 0);
> >   set_task_cpu(next_task, lowest_rq->cpu);
> >     WARN_ON_ONCE(is_migration_disabled(p));
> >       ---------OOPS-------------
>
> I don't see how this can happen.
>
> -- Steve

--
BRs
Schspa Shi