Re: [PATCH sched_ext/for-7.2] sched_ext: Document the ops compat strategy in compat.h/compat.bpf.h

[Andrea Righi <arighi@nvidia.com>]

Hi Tejun,

On Sun, Apr 19, 2026 at 06:18:52AM -1000, Tejun Heo wrote:
> The comments around SCX_OPS_DEFINE() and SCX_OPS_OPEN() were vague about
> how backward compatibility actually works. Expand them to describe the two
> mechanisms: load-time BTF fix-up for additive changes, and multi-variant
> struct_ops for incompatible ones.

Thanks for clarifying these comments.

Acked-by: Andrea Righi <arighi@nvidia.com>

-Andrea

> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> ---
>  tools/sched_ext/include/scx/compat.bpf.h |    6 ++++--
>  tools/sched_ext/include/scx/compat.h     |   23 +++++++++++++++++++----
>  2 files changed, 23 insertions(+), 6 deletions(-)
> 
> --- a/tools/sched_ext/include/scx/compat.bpf.h
> +++ b/tools/sched_ext/include/scx/compat.bpf.h
> @@ -423,8 +423,10 @@ static inline void scx_bpf_dsq_reenq(u64
>  }
>  
>  /*
> - * Define sched_ext_ops. This may be expanded to define multiple variants for
> - * backward compatibility. See compat.h::SCX_OPS_LOAD/ATTACH().
> + * Define sched_ext_ops. See compat.h::SCX_OPS_OPEN() for how backward
> + * compatibility is handled (this macro can be expanded to emit multiple
> + * variants for incompatible op changes; SCX_OPS_OPEN() handles purely
> + * additive changes at load time).
>   */
>  #define SCX_OPS_DEFINE(__name, ...)						\
>  	SEC(".struct_ops.link")							\
> --- a/tools/sched_ext/include/scx/compat.h
> +++ b/tools/sched_ext/include/scx/compat.h
> @@ -149,10 +149,24 @@ static inline long scx_hotplug_seq(void)
>  }
>  
>  /*
> - * struct sched_ext_ops can change over time. If compat.bpf.h::SCX_OPS_DEFINE()
> - * is used to define ops and compat.h::SCX_OPS_LOAD/ATTACH() are used to load
> - * and attach it, backward compatibility is automatically maintained where
> - * reasonable.
> + * Open the sched_ext_ops skeleton.
> + *
> + * struct sched_ext_ops can change over time. Two complementary mechanisms
> + * keep BPF schedulers built against newer headers running on older kernels:
> + *
> + * 1. Load-time fix-up (this macro). For each optional ops callback or field
> + *    added to struct sched_ext_ops, an explicit stanza below probes the
> + *    running kernel's BTF via __COMPAT_struct_has_field() and, if the field
> + *    is missing, clears it in the in-memory struct_ops (with a warning to
> + *    stderr) before load. Handles additive changes - a new stanza must be
> + *    added here for each new optional field.
> + *
> + * 2. Multi-variant struct_ops via compat.bpf.h::SCX_OPS_DEFINE(). That
> + *    macro can be expanded to emit several variants of struct sched_ext_ops,
> + *    and SCX_OPS_LOAD()/ATTACH() can pick the right one based on what the
> + *    kernel supports. Needed when an existing operation has to change
> + *    incompatibly (e.g. a callback signature changes); the load-time
> + *    fix-up above only handles purely additive changes.
>   *
>   * ec7e3b0463e1 ("implement-ops") in https://github.com/sched-ext/sched_ext is
>   * the current minimum required kernel version.
> @@ -225,6 +239,7 @@ static inline void __scx_ops_assoc_prog(
>  }
>  #endif
>  
> +/* See SCX_OPS_OPEN() above for backward-compatibility handling. */
>  #define SCX_OPS_LOAD(__skel, __ops_name, __scx_name, __uei_name) ({		\
>  	struct bpf_program *__prog;						\
>  	UEI_SET_SIZE(__skel, __ops_name, __uei_name);				\