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sched_ext: Document the ops compat strategy in compat.h/compat.bpf.h

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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.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Andrea Righi <arighi@nvidia.com>
Acked-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
This commit is contained in:
Tejun Heo 2026-04-19 06:18:52 -10:00
parent 5425abc222
commit e613cc2361
2 changed files with 23 additions and 6 deletions
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6
tools/sched_ext/include/scx/compat.bpf.h
View File

@ -423,8 +423,10 @@ static inline void scx_bpf_dsq_reenq(u64 dsq_id, u64 reenq_flags)
}
/*
* 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") \

23
tools/sched_ext/include/scx/compat.h
View File

@ -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(struct bpf_program *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); \