Re: [PATCH 5/6] sched_ext: Add a kfunc to read a task's preferred NUMA node

[K Prateek Nayak]

Hello Andrea,

On 10/27/2026 9:43 AM, Andrea Righi wrote:
> +__bpf_kfunc s32 scx_bpf_task_numa_nid(const struct task_struct *p)
> +{
> +#ifdef CONFIG_NUMA_BALANCING
> +	if (task_on_scx(p)) {
> +		struct scx_sched *sch;
> +
> +		guard(rcu)();
> +		sch = scx_task_sched_rcu(p);
> +		if (!sch || !SCX_HAS_OP(sch, set_numa_node))
> +			return NUMA_NO_NODE;
> +	}
> +
> +	return READ_ONCE(p->numa_preferred_nid);

Would the ext scheduler need the raw numa_preferred_nid when
sched_numa_balancing is disabled?

It may just be pointing to some stale value if NUMA balancing
was disabled at runtime right?

> +#else
> +	return NUMA_NO_NODE;
> +#endif
> +}
> +
>  /**
>   * scx_bpf_locked_rq - Return the rq currently locked by SCX
>   * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
> @@ -11836,6 +11877,7 @@ BTF_ID_FLAGS(func, scx_bpf_put_cpumask, KF_RELEASE)
>  BTF_ID_FLAGS(func, scx_bpf_task_running, KF_RCU)

On a tangential note, scx_bpf_task_running() just does a
rcu_access_pointer() (resolves to a READ_ONCE()) and doesn't take a
rcu_read_lock() so does it really need KF_RCU?

>  BTF_ID_FLAGS(func, scx_bpf_task_cpu, KF_RCU)
>  BTF_ID_FLAGS(func, scx_bpf_task_cid, KF_RCU)
> +BTF_ID_FLAGS(func, scx_bpf_task_numa_nid, KF_RCU)
>  BTF_ID_FLAGS(func, scx_bpf_locked_rq, KF_IMPLICIT_ARGS | KF_RET_NULL)
>  BTF_ID_FLAGS(func, scx_bpf_cpu_curr, KF_IMPLICIT_ARGS | KF_RET_NULL | KF_RCU_PROTECTED)
>  BTF_ID_FLAGS(func, scx_bpf_cid_curr, KF_IMPLICIT_ARGS | KF_RET_NULL | KF_RCU_PROTECTED)

-- 
Thanks and Regards,
Prateek