Re: [PATCH sched_ext/for-7.1] tools/sched_ext: Add __COMPAT wrapper for scx_bpf_sub_dispatch()

[Andrea Righi <arighi@nvidia.com>]

Hi Cheng-Yang,

On Sun, Mar 22, 2026 at 10:44:23PM +0800, Cheng-Yang Chou wrote:
> Add __COMPAT_scx_bpf_sub_dispatch() to compat.bpf.h so that schedulers
> using sub-sched dispatch can build and run on older kernels that lack the
> kfunc. Guard the call with CONFIG_EXT_SUB_SCHED and bpf_ksym_exists(),
> falling back to false when unavailable.
> 
> Update scx_qmap to use the new wrapper instead of calling
> scx_bpf_sub_dispatch() directly.
> 
> Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
> ---
>  tools/sched_ext/include/scx/compat.bpf.h | 13 +++++++++++++
>  tools/sched_ext/scx_qmap.bpf.c           |  2 +-
>  2 files changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/sched_ext/include/scx/compat.bpf.h b/tools/sched_ext/include/scx/compat.bpf.h
> index 83b3425e63b2..f24b9a1c75ad 100644
> --- a/tools/sched_ext/include/scx/compat.bpf.h
> +++ b/tools/sched_ext/include/scx/compat.bpf.h
> @@ -108,6 +108,19 @@ static inline struct task_struct *__COMPAT_scx_bpf_dsq_peek(u64 dsq_id)
>  	return p;
>  }
>  
> +/*
> + * v7.1: scx_bpf_sub_dispatch() for sub-sched dispatch. Preserve until we drop
> + * the compat layer for older kernels that lack the kfunc.
> + */
> +static inline bool __COMPAT_scx_bpf_sub_dispatch(u64 cgroup_id)
> +{
> +#ifdef CONFIG_EXT_SUB_SCHED
> +	if (bpf_ksym_exists(scx_bpf_sub_dispatch))
> +		return scx_bpf_sub_dispatch(cgroup_id);
> +#endif

Isn't bpf_ksym_exists() enough? The CONFIG_EXT_SUB_SCHED guard takes effect
only when we build an scx scheduler using __COMPAT_scx_bpf_sub_dispatch(),
instead we want to build the scheduler anywhere and then the BPF verifier
should evaluate the availability of scx_bpf_sub_dispatch().

Then the symbol will be resolved by this (from include/scx/common.bpf.h):

 bool scx_bpf_sub_dispatch(u64 cgroup_id) __ksym __weak;

So, IIUC the #ifdef CONFIG_EXT_SUB_SCHED guard is not really needed here.

Thanks,
-Andrea

> +	return false;
> +}
> +
>  /**
>   * __COMPAT_is_enq_cpu_selected - Test if SCX_ENQ_CPU_SELECTED is on
>   * in a compatible way. We will preserve this __COMPAT helper until v6.16.
> diff --git a/tools/sched_ext/scx_qmap.bpf.c b/tools/sched_ext/scx_qmap.bpf.c
> index f3587fb709c9..dd44d8db0341 100644
> --- a/tools/sched_ext/scx_qmap.bpf.c
> +++ b/tools/sched_ext/scx_qmap.bpf.c
> @@ -490,7 +490,7 @@ void BPF_STRUCT_OPS(qmap_dispatch, s32 cpu, struct task_struct *prev)
>  
>  	for (i = 0; i < MAX_SUB_SCHEDS; i++) {
>  		if (sub_sched_cgroup_ids[i] &&
> -		    scx_bpf_sub_dispatch(sub_sched_cgroup_ids[i]))
> +		    __COMPAT_scx_bpf_sub_dispatch(sub_sched_cgroup_ids[i]))
>  			return;
>  	}
>  
> -- 
> 2.48.1
>