Re: [PATCH sched_ext/for-7.4] sched_ext: Keep proxy donors with slice left on the local DSQ

[Tejun Heo <tj@kernel.org>]

Hello, Andrea.

On Thu, Oct 01, 2026 at 09:12:16PM +0200, Andrea Righi wrote:
> +			if ((p->scx.flags & SCX_TASK_IMMED) && !proxy_put) {
>  				p->scx.flags |= SCX_TASK_REENQ_PREEMPTED;
>  				scx_do_enqueue_task(rq, p, SCX_ENQ_REENQ, -1);

I wonder whether IMMED should matter for a blocked donor at all. IMMED
should trigger a reenqueue iff the task is not being serviced by a CPU. A
donor under proxy execution is being serviced. The CPU is working on its
behalf and there's nothing the scheduler can do to make it go faster by
placing it elsewhere. If so, a donor with slice left can stay on the local
DSQ regardless of IMMED and of which put this is, and
SCX_RQ_PROXY_PICK_PENDING isn't needed. The deferred local check would need
the same exemption as the task stays SCX_TASK_IMMED.

> +		/* Delegate retained donor admission to its owning BPF scheduler. */
> +		if (p->is_blocked) {
> +			if (WARN_ON_ONCE(!sch))
> +				goto switch_class;
> +			WARN_ON_ONCE(!(sch->ops.flags & SCX_OPS_ENQ_BLOCKED));
> +			scx_do_enqueue_task(rq, p, 0, -1);
> +			goto switch_class;
> +		}

Can this be folded into the following block? scx_do_enqueue_task() already
adds SCX_ENQ_BLOCKED, so a !p->is_blocked test on the ENQ_LAST condition
would route donors to the plain enqueue.

Thanks.

--
tejun