Re: [PATCH 3/3] sched_ext: Make scx_bpf_kick_cid() return s32

[Andrea Righi <arighi@nvidia.com>]

Hi Tejun,

On Wed, Jun 03, 2026 at 07:21:53AM -1000, Tejun Heo wrote:
> Switch scx_bpf_kick_cid() from void to s32 so future cap enforcement can
> surface failures. cid interface is introduced in this cycle and has no
> external users, so the ABI change is safe. Subsequent patches will add
> -EPERM returns when the calling sub-sched lacks the required cap on the
> target cid.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> ---
>  kernel/sched/ext.c                       | 13 ++++++++-----
>  tools/sched_ext/include/scx/common.bpf.h |  2 +-
>  2 files changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 32ebbc351564..fedd501de67e 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -9405,9 +9405,10 @@ __bpf_kfunc void scx_bpf_kick_cpu(s32 cpu, u64 flags, const struct bpf_prog_aux
>   * @flags: %SCX_KICK_* flags
>   * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
>   *
> - * cid-addressed equivalent of scx_bpf_kick_cpu().
> + * cid-addressed equivalent of scx_bpf_kick_cpu(). Return 0 on success,
> + * -errno otherwise.
>   */
> -__bpf_kfunc void scx_bpf_kick_cid(s32 cid, u64 flags, const struct bpf_prog_aux *aux)
> +__bpf_kfunc s32 scx_bpf_kick_cid(s32 cid, u64 flags, const struct bpf_prog_aux *aux)
>  {
>  	struct scx_sched *sch;
>  	s32 cpu;
> @@ -9415,10 +9416,12 @@ __bpf_kfunc void scx_bpf_kick_cid(s32 cid, u64 flags, const struct bpf_prog_aux
>  	guard(rcu)();
>  	sch = scx_prog_sched(aux);
>  	if (unlikely(!sch))
> -		return;
> +		return -ENODEV;
>  	cpu = scx_cid_to_cpu(sch, cid);
> -	if (cpu >= 0)
> -		scx_kick_cpu(sch, cpu, flags);
> +	if (cpu < 0)
> +		return -EINVAL;

Right now scx_cid_to_cpu() can only return -EINVAL, but should we propagate the
error to the caller and return cpu here (in case we add more errors in the
future)?

Thanks,
-Andrea