Re: [PATCH] tools/sched_ext: Handle pinned tasks in scx_central

[Andrea Righi <arighi@nvidia.com>]

Hi Cheng-Yang,

On Tue, Apr 14, 2026 at 11:45:59AM +0800, Cheng-Yang Chou wrote:
...
> @@ -155,6 +166,13 @@ static bool dispatch_to_cpu(s32 cpu)
>  		if (!bpf_cpumask_test_cpu(cpu, p->cpus_ptr)) {
>  			__sync_fetch_and_add(&nr_mismatches, 1);
>  			scx_bpf_dsq_insert(p, FALLBACK_DSQ_ID, SCX_SLICE_INF, 0);
> +
> +			/*
> +			 * Kick the task's home CPU to pick it up from the
> +			 * fallback DSQ promptly.
> +			 */
> +			scx_bpf_kick_cpu(scx_bpf_task_cpu(p), SCX_KICK_PREEMPT);
> +

You can also pass SCX_ENQ_PREEMPT to scx_bpf_dsq_insert() and avoid calling
scx_bpf_kick_cpu().

More in general, do we really need to preempt the CPU here? It'll be preempted
in the central_timerfn() handler later, right? If we preempt the CPU here we may
have lots of preemption events when many affinitized tasks are running (i.e.,
stress-ng --race-sched 0).

Thanks,
-Andrea

>  			bpf_task_release(p);
>  			/*
>  			 * We might run out of dispatch buffer slots if we continue dispatching
> -- 
> 2.48.1
>