Re: [PATCH 14/18] sched_ext: Split curr|donor references properly

[Peter Zijlstra]

On Mon, Aug 31, 2026 at 03:42:24PM +0200, Andrea Righi wrote:

> Consider FAIR and EXT tasks with sched_ext running in partial mode. FAIR
> can be replaced with a higher scheduling class such as RT or deadline
> without changing the class interaction described here. The possible
> combinations are:
> 
>   1. D is EXT, O is EXT, T is EXT
> 
>      D can interrupt T according to BPF scheduling policy. O executes
>      with D's EXT priority and runtime budget, while T waits in EXT.
> 
>   2. D is EXT, O is EXT, T is FAIR
> 
>      D is visible to the BPF scheduler, but cannot preempt T because
>      EXT is below FAIR. Once T stops, BPF can dispatch D and O executes
>      with D's EXT priority and runtime budget. If T becomes runnable
>      again, it preempts the D/O proxy execution.
> 
>   3. D is EXT, O is FAIR, T is EXT
> 
>      This cannot represent T preempting O because EXT is below FAIR.
> 
>   4. D is EXT, O is FAIR, T is FAIR
> 
>      D cannot boost O above T because EXT is below FAIR. O and T
>      continue competing under FAIR. Once O releases M, D wakes and
>      resumes normal EXT scheduling.
> 
>   5. D is FAIR, O is EXT, T is EXT
> 
>      D preempts T as the higher-class scheduling context. O executes
>      with D's FAIR priority and runtime budget, while T waits in EXT.
>      D is not visible to the BPF scheduler.
> 
>   6. D is FAIR, O is EXT, T is FAIR
> 
>      D competes with T according to its FAIR deadline. When D is
>      selected, O executes with D's FAIR priority and runtime budget.
>      D is not visible to the BPF scheduler.
> 
>   7. D is FAIR, O is FAIR, T is EXT
> 
>      This cannot represent T preempting O because EXT is below FAIR.
> 
>   8. D is FAIR, O is FAIR, T is FAIR
> 
>      O, T, and D all have FAIR scheduling contexts. D remains runnable
>      as a blocked proxy donor. When CFS selects D, O executes using D's
>      FAIR scheduling context. When CFS selects O, O executes using its
>      own FAIR context, and when CFS selects T, T executes normally. D
>      is not visible to the BPF scheduler.

I don't see how 3,4 can happen. If there is a runnable FAIR task, then
pick will pick that directly.

Per the pick priority, the donor class must always be higher than the
owner class.