Re: [PATCH v4 2/3] sched_ext: Fix cpu_released while RT task and SCX task are scheduled concurrently

[Peter Zijlstra <peterz@infradead.org>]

Could you please not thread your new patches onto the old thread? That
makes them near impossible to find.

On Tue, Aug 19, 2025 at 06:55:38AM +0000, liuwenfang wrote:
> Supposed RT task(RT1) is running on CPU0 and RT task(RT2) is awakened on CPU1,
> RT1 becomes sleep and SCX task(SCX1) will be dispatched to CPU0, RT2 will be
> placed on CPU0:
> 
> CPU0(schedule)                                     CPU1(try_to_wake_up)
> set_current_state(TASK_INTERRUPTIBLE)              try_to_wake_up # RT2
> __schedule                                           select_task_rq # CPU0 is selected
> LOCK rq(0)->lock # lock CPU0 rq                        ttwu_queue
>   deactivate_task # RT1                                  LOCK rq(0)->lock # busy waiting
>     pick_next_task # no more RT tasks on rq                 |
>       prev_balance                                          |
>         balance_scx                                         |
>           balance_one                                       |
>             rq->scx.cpu_released = false;                   |
>               consume_global_dsq                            |
>                 consume_dispatch_q                          |
>                   consume_remote_task                       |
>                     UNLOCK rq(0)->lock                      V
>                                                          LOCK rq(0)->lock # succ
>                     deactivate_task # SCX1               ttwu_do_activate
>                     LOCK rq(0)->lock # busy waiting      activate_task # RT2 equeued
>                        |                                 UNLOCK rq(0)->lock
>                        V
>                     LOCK rq(0)->lock # succ
>                     activate_task # SCX1
>       pick_task # RT2 is picked
>       put_prev_set_next_task # prev is RT1, next is RT2, rq->scx.cpu_released = false;
> UNLOCK rq(0)->lock
> 
> At last, RT2 will be running on CPU0 with rq->scx.cpu_released being false, which would
> lead the BPF scheduler to make decisions improperly.
> 
> So, check the sched class in __put_prev_set_next_scx() to fix the value of
> rq->scx.cpu_released.

Oh gawd, this is terrible.

Why would you start the pick in balance and then cry when you fail the
pick in pick ?!?

This is also the reason you need that weird CLASS_EXT exception in
prev_balance(), isn't it?

You're now asking for a 3rd call out to do something like:

  ->balance() -- ready a task for pick
  ->pick() -- picks a random other task
  ->put_prev() -- oops, our task didn't get picked, stick it back

Which is bloody ludicrous. So no. We're not doing this.

Why can't pick DTRT ?