Re: [PATCH 02/10] sched_ext: Add select_cpu kfuncs to scx_kfunc_ids_unlocked

[Andrea Righi]

On Thu, Apr 09, 2026 at 08:30:38PM -1000, Tejun Heo wrote:
> select_cpu_from_kfunc() has an extra scx_kf_allowed_if_unlocked() branch
> that accepts calls from unlocked contexts and takes task_rq_lock() itself
> - a "callable from unlocked" property encoded in the kfunc body rather
> than in set membership. That's fine while the runtime check is the
> authoritative gate, but the upcoming verifier-time filter uses set
> membership as the source of truth and needs it to reflect every context
> the kfunc may be called from.
> 
> Add the three select_cpu kfuncs to scx_kfunc_ids_unlocked so their full
> set of callable contexts is captured by set membership. This follows the
> existing dual-set convention used by scx_bpf_dsq_move{,_vtime} and
> scx_bpf_dsq_move_set_{slice,vtime}, which are members of both
> scx_kfunc_ids_dispatch and scx_kfunc_ids_unlocked.
> 
> While at it, add brief comments on each duplicate BTF_ID_FLAGS block
> (including the pre-existing dsq_move ones) explaining the dual
> membership.
> 
> No runtime behavior change: the runtime check in select_cpu_from_kfunc()
> remains the authoritative gate until it is removed along with the rest
> of the scx_kf_mask enforcement in a follow-up.
> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>

Reviewed-by: Andrea Righi <arighi@xxxxxxxxxx>

Minor nit below.

> ---
>  kernel/sched/ext.c      | 6 ++++++
>  kernel/sched/ext_idle.c | 4 ++++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index b757b853b42b..cf441fb4b1ad 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -8497,6 +8497,7 @@ BTF_ID_FLAGS(func, scx_bpf_dispatch_nr_slots, KF_IMPLICIT_ARGS)
>  BTF_ID_FLAGS(func, scx_bpf_dispatch_cancel, KF_IMPLICIT_ARGS)
>  BTF_ID_FLAGS(func, scx_bpf_dsq_move_to_local, KF_IMPLICIT_ARGS)
>  BTF_ID_FLAGS(func, scx_bpf_dsq_move_to_local___v2, KF_IMPLICIT_ARGS)
> +/* also in scx_kfunc_ids_unlocked: also callable from unlocked contexts */
>  BTF_ID_FLAGS(func, scx_bpf_dsq_move_set_slice, KF_RCU)
>  BTF_ID_FLAGS(func, scx_bpf_dsq_move_set_vtime, KF_RCU)
>  BTF_ID_FLAGS(func, scx_bpf_dsq_move, KF_RCU)

Nit: we don't see it from the patch context, but below there's
scx_bpf_sub_dispatch() which also seems to be callable from unlocked context
with this comment. Maybe move scx_bpf_sub_dispatch() before this comment?

> @@ -8612,10 +8613,15 @@ __bpf_kfunc_end_defs();
>  
>  BTF_KFUNCS_START(scx_kfunc_ids_unlocked)
>  BTF_ID_FLAGS(func, scx_bpf_create_dsq, KF_IMPLICIT_ARGS | KF_SLEEPABLE)
> +/* also in scx_kfunc_ids_dispatch: also callable from ops.dispatch() */
>  BTF_ID_FLAGS(func, scx_bpf_dsq_move_set_slice, KF_RCU)
>  BTF_ID_FLAGS(func, scx_bpf_dsq_move_set_vtime, KF_RCU)
>  BTF_ID_FLAGS(func, scx_bpf_dsq_move, KF_RCU)
>  BTF_ID_FLAGS(func, scx_bpf_dsq_move_vtime, KF_RCU)
> +/* also in scx_kfunc_ids_select_cpu: also callable from ops.select_cpu()/ops.enqueue() */
> +BTF_ID_FLAGS(func, __scx_bpf_select_cpu_and, KF_IMPLICIT_ARGS | KF_RCU)
> +BTF_ID_FLAGS(func, scx_bpf_select_cpu_and, KF_RCU)
> +BTF_ID_FLAGS(func, scx_bpf_select_cpu_dfl, KF_IMPLICIT_ARGS | KF_RCU)
>  BTF_KFUNCS_END(scx_kfunc_ids_unlocked)
>  
>  static const struct btf_kfunc_id_set scx_kfunc_set_unlocked = {
> diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
> index cd88aee47bd8..8c31fb65477c 100644
> --- a/kernel/sched/ext_idle.c
> +++ b/kernel/sched/ext_idle.c
> @@ -1482,6 +1482,10 @@ static const struct btf_kfunc_id_set scx_kfunc_set_idle = {
>   * contexts where @p's pi_lock state is unknown. Keep them out of
>   * BPF_PROG_TYPE_TRACING by registering them in their own set which is exposed
>   * only to STRUCT_OPS and SYSCALL programs.
> + *
> + * These kfuncs are also members of scx_kfunc_ids_unlocked (see ext.c) because
> + * they're callable from unlocked contexts in addition to ops.select_cpu() and
> + * ops.enqueue().
>   */
>  BTF_KFUNCS_START(scx_kfunc_ids_select_cpu)
>  BTF_ID_FLAGS(func, __scx_bpf_select_cpu_and, KF_IMPLICIT_ARGS | KF_RCU)
> -- 
> 2.53.0
> 

Thanks,
-Andrea