[PATCH sched_ext/for-7.4] tools/sched_ext: Skip scx_lib_init_probe() without CONFIG_FUNCTION_TRACER

[PATCH sched_ext/for-7.4] tools/sched_ext: Skip scx_lib_init_probe() without CONFIG_FUNCTION_TRACER

[Cheng-Yang Chou]

scx_lib_init_probe() is a fentry probe auto-attached by every scheduler
via SCX_OPS_LOAD()/SCX_OPS_ATTACH(). Without CONFIG_FUNCTION_TRACER it
can't attach, and skel__attach()'s all-or-nothing semantics takes the
whole scheduler down.

Skip its autoload when /proc/sys/kernel/ftrace_enabled is absent, and
skip disabled programs in the post-load struct_ops association loop so
it doesn't try to associate a program with no FD. Reproduced and fixed
in vng with a CONFIG_FUNCTION_TRACER=n kernel.

Syncs the fix from https://github.com/sched-ext/scx/pull/3758. As
discussed in GitHub, once we stop supporting pre-6.18 kernels, this
workaround can be removed.

Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
---
 tools/sched_ext/include/scx/compat.h | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/tools/sched_ext/include/scx/compat.h b/tools/sched_ext/include/scx/compat.h
index 7c12df45fdba..faf608cbed55 100644
--- a/tools/sched_ext/include/scx/compat.h
+++ b/tools/sched_ext/include/scx/compat.h
@@ -361,13 +361,27 @@ static inline void __scx_ops_assoc_prog(struct bpf_program *prog,
 }
 #endif
 
+/*
+ * Whether the kernel supports function tracing (CONFIG_FUNCTION_TRACER),
+ * needed for fentry/fexit BPF programs to load or attach.
+ * /proc/sys/kernel/ftrace_enabled only exists when it's compiled in, so its
+ * presence is a cheap proxy.
+ */
+static inline bool __COMPAT_function_tracer_available(void)
+{
+	return access("/proc/sys/kernel/ftrace_enabled", F_OK) == 0;
+}
+
 /* 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);				\
+	if (!__COMPAT_function_tracer_available())				\
+		bpf_program__set_autoload((__skel)->progs.scx_lib_init_probe, false); \
 	SCX_BUG_ON(__scx_name##__load((__skel)), "Failed to load skel");	\
 	bpf_object__for_each_program(__prog, (__skel)->obj) {			\
-		if (bpf_program__type(__prog) == BPF_PROG_TYPE_STRUCT_OPS)	\
+		if (bpf_program__type(__prog) == BPF_PROG_TYPE_STRUCT_OPS ||	\
+		    !bpf_program__autoload(__prog))				\
 			continue;						\
 		__scx_ops_assoc_prog(__prog, (__skel)->maps.__ops_name,		\
 				     #__ops_name);				\
-- 
2.43.0

Re: [PATCH sched_ext/for-7.4] tools/sched_ext: Skip scx_lib_init_probe() without CONFIG_FUNCTION_TRACER

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The SCX_OPS_LOAD macro introduces a hard compile-time dependency on `scx_lib_init_probe` existing in the user's BPF skeleton, which will cause a C compiler error for schedulers that do not define this probe.
--

--- Patch [1]: [PATCH sched_ext/for-7.4] tools/sched_ext: Skip scx_lib_init_probe() without CONFIG_FUNCTION_TRACER ---
commit fc2df7f154a3b0d6909630ef49d7036552669ee3
Author: Cheng-Yang Chou <yphbchou0911@gmail.com>

tools/sched_ext: Skip scx_lib_init_probe() without CONFIG_FUNCTION_TRACER

This commit updates the SCX_OPS_LOAD macro to conditionally disable
autoload for the scx_lib_init_probe program if function tracing is
unavailable. It also modifies the post-load loop to skip programs that
have autoload disabled.

> diff --git a/tools/sched_ext/include/scx/compat.h b/tools/sched_ext/include/scx/compat.h
> --- a/tools/sched_ext/include/scx/compat.h
> +++ b/tools/sched_ext/include/scx/compat.h

[ ... ]

>  /* 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);				\
> +	if (!__COMPAT_function_tracer_available())				\
> +		bpf_program__set_autoload((__skel)->progs.scx_lib_init_probe, false); \

[Severity: Medium]
Will this cause a compile-time failure for out-of-tree schedulers that do
not include a scx_lib_init_probe program?

Skeletons generated by bpftool only contain fields for BPF programs that
are actually present in the BPF object. If a user's scheduler skeleton
lacks this specific probe, (__skel)->progs.scx_lib_init_probe won't exist
in the generated struct and will trigger a C compiler error when invoking
this macro.

Could this look up the program dynamically using
bpf_object__find_program_by_name() at runtime instead, to avoid breaking
compilation for schedulers without this probe?

>  	SCX_BUG_ON(__scx_name##__load((__skel)), "Failed to load skel");	\

[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260824155051.20841-1-yphbchou0911@gmail.com?part=1