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

[Tejun Heo <tj@kernel.org>]

Hello,

On Tue, Apr 14, 2026 at 11:45:59AM +0800, Cheng-Yang Chou wrote:
> diff --git a/tools/sched_ext/scx_central.bpf.c b/tools/sched_ext/scx_central.bpf.c
> index 4efcce099bd5..67b160a7ce18 100644
> --- a/tools/sched_ext/scx_central.bpf.c
> +++ b/tools/sched_ext/scx_central.bpf.c
> @@ -96,7 +96,13 @@ s32 BPF_STRUCT_OPS(central_select_cpu, struct task_struct *p,
>  	 * disturbing other CPUs. It's safe to blindly return the central cpu as
>  	 * select_cpu() is a hint and if @p can't be on it, the kernel will
>  	 * automatically pick a fallback CPU.
> +	 *
> +	 * For tasks pinned to a single CPU, return the current CPU to avoid
> +	 * an unnecessary fallback.
>  	 */
> +	if (p->nr_cpus_allowed == 1)
> +		return scx_bpf_task_cpu(p);
> +

This isn't necessary. select_cpu() isn't called if nr_cpus_allowed == 1.

> @@ -107,12 +113,17 @@ 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 or with migration disabled 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.
> +	 *
> +	 * p->nr_cpus_allowed is not updated until migrate_disable_switch()
> +	 * runs during context switch, which happens after ops.enqueue(). Check
> +	 * is_migration_disabled() directly to catch tasks in this window.
>  	 */
> -	if ((p->flags & PF_KTHREAD) && p->nr_cpus_allowed == 1) {
> +	if (p->nr_cpus_allowed == 1 || is_migration_disabled(p)) {

Let's not expand the condition here. This is to guarantee forward progress
of the scheduler itself.

> @@ -155,6 +166,13 @@ static bool dispatch_to_cpu(s32 cpu)
>  		if (!bpf_cpumask_test_cpu(cpu, p->cpus_ptr)) {

Instead, can you expand the condition above to "if this task can't run on
the target CPU for whatever reason"?

Thanks.

-- 
tejun