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

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 <[email protected]>
---
 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