Re: [PATCH 18/39] sched_ext: Allow BPF schedulers to disallow specific tasks from joining SCHED_EXT

[Tejun Heo]

Hello, Peter.

On Mon, Jun 24, 2024 at 02:40:53PM +0200, Peter Zijlstra wrote:
> On Wed, May 01, 2024 at 05:09:53AM -1000, Tejun Heo wrote:
> > BPF schedulers might not want to schedule certain tasks - e.g. kernel
> > threads. This patch adds p->scx.disallow which can be set by BPF schedulers
> > in such cases. The field can be changed anytime and setting it in
> > ops.prep_enable() guarantees that the task can never be scheduled by
> > sched_ext.
> 
> Why ?!?!
> 
> By leaving kernel threads fair, and fair sitting above the BPF thing,
> it is not dissimilar to promoting them to FIFO. They will instantly
> preempt the BPF thing and keep running for as long as they need. The
> only real difference between this and actual FIFO is the behaviour on
> contention.

Yes, from sched_ext's POV, in partial mode, CFS isn't all that different
from FIFO. Whenever there are tasks to run in CFS, CPUs are taken away.
Right now, partial mode can be useful for leaving a part of system on CFS
(e.g. in a cpuset partitioned system), when the scheduler is narrowly
focused and doesn't cover everything necessary (e.g. EAS).

> This seems like a very bad thing to have, and your 'changelog' has no
> justification what so ever.

This is a bit of duplicate interface in that in partial mode sched_ext can
already be opted in by setting per-thread sched class. However, some use
cases wanted this so that the BPF scheduler has the final say over who can
be on it rather than the userspace. It's a convenience feature for some use
cases.

Thanks.

-- 
tejun