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

['Tejun Heo' <tj@kernel.org>]

Hello,

On Wed, Aug 20, 2025 at 11:18:10AM +0200, Peter Zijlstra wrote:
> >   This guarantee changes if we add @rf to pick_task() and let it unlock and
> >   relock. A higher priority task may get queued while the rq lock is
> >   released and then the lower priority pick_task() may still return a task
> >   of its own.
> 
> No, this would be broken. This guarantee must not change.
> 
> What you can do however is something like:
> 
> again:
>    p = pick_local_task();
>    if (!p) {
>      unlock(rq, rf);
>      // get yourself a local task
>      lock(rq, rf);
>      if (higher-class-task-available(rq)) {
>        // roll back whatever state
>        return RETRY_TASK;
>      }
>      goto again;
>    }
> 
>    return p;

Isn't that kinda nasty as we'd have to do that in every pick_task(). What'd
be equivalent but in a central location would be making sure that
wakeup_preempt() asserts a resched event if it hits a race window like that.
I haven't thought too much about how that should be done exactly but the
races should be pretty rare, so it'd be surprising if the behavior
difference is noticeable.

Thanks.

-- 
tejun