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

[Kuba Piecuch <jpiecuch@google.com>]

Hi Cheng-Yang,

On Mon Apr 13, 2026 at 11:04 AM UTC, Cheng-Yang Chou wrote:
> @@ -107,12 +113,13 @@ void BPF_STRUCT_OPS(central_enqueue, struct task_struct *p, u64 enq_flags)
>  	__sync_fetch_and_add(&nr_total, 1);
>  
>  	/*
> -	 * Push per-cpu kthreads at the head of local dsq's and preempt the
> -	 * corresponding CPU. This ensures that e.g. ksoftirqd isn't blocked
> -	 * behind other threads which is necessary for forward progress
> -	 * guarantee as we depend on the BPF timer which may run from ksoftirqd.
> +	 * Push tasks pinned to a single CPU at the head of local dsq's and
> +	 * preempt the corresponding CPU. This ensures that e.g. ksoftirqd
> +	 * isn't blocked behind other threads which is necessary for forward
> +	 * progress guarantee as we depend on the BPF timer which may run
> +	 * from ksoftirqd.
>  	 */
> -	if ((p->flags & PF_KTHREAD) && p->nr_cpus_allowed == 1) {
> +	if (p->nr_cpus_allowed == 1) {

If you want to cover all cases, then I think you should also check
is_migration_disabled(p). When a task disables migration, it is not reflected
in p->cpus_ptr or p->nr_cpus_allowed until the task is switched out (see
migrate_disable_switch()), which happens __after__ ops.enqueue().
So here, you could see p->nr_cpus_allowed > 1 even though the task has
migration disabled.

Thanks,
Kuba