Re: [PATCH 12/14] sched: Add shared runqueue locking to __task_rq_lock()

[Tejun Heo <tj@kernel.org>]

Hello, again.

On Tue, Sep 16, 2025 at 12:29:57PM -1000, Tejun Heo wrote:
...
> Long term, I think maintaining flexibility is of higher importance for
> sched_ext than e.g. small performance improvements or even design or
> implementation aesthetics. The primary purpose is enabling trying out new,
> sometimes wild, things after all. As such, I don't think it'd be a good idea
> to put strict restrictions on how the BPF side operates unless it affects
> the ability to recover the system from a malfunctioning BPF scheduler, of
> course.

Thinking a bit more about it. I wonder the status-quo is actually an okay
balance. All in-kernel sched classes are per-CPU rich rq design, which
meshes well with the current locking scheme, for obvious reasons.

sched_ext is an oddball in that it may want to hot-migrate tasks at the last
minute because who knows what the BPF side wants to do. However, this just
boils down to having to always call balance() before any pick_task()
attempts (including DL server case). Yeah, it's a niggle, especially as
there needs to be a secondary hook to handle losing the race between
balance() and pick_task(), but it's pretty contained conceptually and not a
lot of code.

Thanks.

-- 
tejun