[RFC PATCH v4 1/4] tracing: Introduce tracepoint extended structure

[Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>]

Shrink the struct tracepoint size from 80 bytes to 72 bytes on x86-64 by
moving the (typically NULL) regfunc/unregfunc pointers to an extended
structure.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Michael Jeanson <mjeanson@xxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Yonghong Song <yhs@xxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
Cc: bpf@xxxxxxxxxxxxxxx
Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Cc: Jordan Rife <jrife@xxxxxxxxxx>
---
 include/linux/tracepoint-defs.h |  8 ++++++--
 include/linux/tracepoint.h      | 19 +++++++++++++------
 kernel/tracepoint.c             |  9 ++++-----
 3 files changed, 23 insertions(+), 13 deletions(-)

diff --git a/include/linux/tracepoint-defs.h b/include/linux/tracepoint-defs.h
index 60a6e8314d4c..967c08d9da84 100644
--- a/include/linux/tracepoint-defs.h
+++ b/include/linux/tracepoint-defs.h
@@ -29,6 +29,11 @@ struct tracepoint_func {
 	int prio;
 };
 
+struct tracepoint_ext {
+	int (*regfunc)(void);
+	void (*unregfunc)(void);
+};
+
 struct tracepoint {
 	const char *name;		/* Tracepoint name */
 	struct static_key_false key;
@@ -36,9 +41,8 @@ struct tracepoint {
 	void *static_call_tramp;
 	void *iterator;
 	void *probestub;
-	int (*regfunc)(void);
-	void (*unregfunc)(void);
 	struct tracepoint_func __rcu *funcs;
+	struct tracepoint_ext *ext;
 };
 
 #ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 0dc67fad706c..83dc24ee8b13 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -302,7 +302,7 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
  * structures, so we create an array of pointers that will be used for iteration
  * on the tracepoints.
  */
-#define DEFINE_TRACE_FN(_name, _reg, _unreg, proto, args)		\
+#define __DEFINE_TRACE_EXT(_name, _ext, proto, args)			\
 	static const char __tpstrtab_##_name[]				\
 	__section("__tracepoints_strings") = #_name;			\
 	extern struct static_call_key STATIC_CALL_KEY(tp_func_##_name);	\
@@ -316,9 +316,9 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 		.static_call_tramp = STATIC_CALL_TRAMP_ADDR(tp_func_##_name), \
 		.iterator = &__traceiter_##_name,			\
 		.probestub = &__probestub_##_name,			\
-		.regfunc = _reg,					\
-		.unregfunc = _unreg,					\
-		.funcs = NULL };					\
+		.funcs = NULL,						\
+		.ext = _ext,						\
+	};								\
 	__TRACEPOINT_ENTRY(_name);					\
 	int __traceiter_##_name(void *__data, proto)			\
 	{								\
@@ -341,8 +341,15 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 	}								\
 	DEFINE_STATIC_CALL(tp_func_##_name, __traceiter_##_name);
 
-#define DEFINE_TRACE(name, proto, args)		\
-	DEFINE_TRACE_FN(name, NULL, NULL, PARAMS(proto), PARAMS(args));
+#define DEFINE_TRACE_FN(_name, _reg, _unreg, _proto, _args)		\
+	struct tracepoint_ext __tracepoint_ext_##_name = {		\
+		.regfunc = _reg,					\
+		.unregfunc = _unreg,					\
+	};								\
+	__DEFINE_TRACE_EXT(_name, &__tracepoint_ext_##_name, PARAMS(_proto), PARAMS(_args));
+
+#define DEFINE_TRACE(_name, _proto, _args)				\
+	__DEFINE_TRACE_EXT(_name, NULL, PARAMS(_proto), PARAMS(_args));
 
 #define EXPORT_TRACEPOINT_SYMBOL_GPL(name)				\
 	EXPORT_SYMBOL_GPL(__tracepoint_##name);				\
diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index 6474e2cf22c9..5658dc92f5b5 100644
--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -278,8 +278,8 @@ static int tracepoint_add_func(struct tracepoint *tp,
 	struct tracepoint_func *old, *tp_funcs;
 	int ret;
 
-	if (tp->regfunc && !static_key_enabled(&tp->key)) {
-		ret = tp->regfunc();
+	if (tp->ext && tp->ext->regfunc && !static_key_enabled(&tp->key)) {
+		ret = tp->ext->regfunc();
 		if (ret < 0)
 			return ret;
 	}
@@ -362,9 +362,8 @@ static int tracepoint_remove_func(struct tracepoint *tp,
 	switch (nr_func_state(tp_funcs)) {
 	case TP_FUNC_0:		/* 1->0 */
 		/* Removed last function */
-		if (tp->unregfunc && static_key_enabled(&tp->key))
-			tp->unregfunc();
-
+		if (tp->ext && tp->ext->unregfunc && static_key_enabled(&tp->key))
+			tp->ext->unregfunc();
 		static_branch_disable(&tp->key);
 		/* Set iterator static call */
 		tracepoint_update_call(tp, tp_funcs);
-- 
2.39.5