mm: mmap_lock: check trace_mmap_lock_$type_enabled() instead of regcount

Since 7d6be67 ("mm: mmap_lock: replace get_memcg_path_buf() with
on-stack buffer") we use trace_mmap_lock_reg()/unreg() only to maintain an
atomic reg_refcount which is checked to avoid performing
get_mm_memcg_path() in case none of the tracepoints using it is enabled.

This can be achieved directly by putting all the work needed for the
tracepoint behind the trace_mmap_lock_##type##_enabled(), as suggested by
Documentation/trace/tracepoints.rst and with the following advantages:

- uses the tracepoint's static key instead of evaluating a branch

- the check tracepoint specific, not shared by all of them

- we can get rid of trace_mmap_lock_reg()/unreg() completely

Thus use the trace_..._enabled() check and remove unnecessary code.

Link: https://lkml.kernel.org/r/20241105113456.95066-2-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Axel Rasmussen <axelrasmussen@google.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/trace/events/mmap_lock.h

@@ -10,9 +10,6 @@
 
 struct mm_struct;
 
-extern int trace_mmap_lock_reg(void);
-extern void trace_mmap_lock_unreg(void);
-
 DECLARE_EVENT_CLASS(mmap_lock,
 
 	TP_PROTO(struct mm_struct *mm, const char *memcg_path, bool write),
@@ -40,16 +37,15 @@ DECLARE_EVENT_CLASS(mmap_lock,
 );
 
 #define DEFINE_MMAP_LOCK_EVENT(name)                                    \
-	DEFINE_EVENT_FN(mmap_lock, name,                                \
+	DEFINE_EVENT(mmap_lock, name,                                   \
 		TP_PROTO(struct mm_struct *mm, const char *memcg_path,  \
 			bool write),                                    \
-		TP_ARGS(mm, memcg_path, write),                         \
-		trace_mmap_lock_reg, trace_mmap_lock_unreg)
+		TP_ARGS(mm, memcg_path, write))
 
 DEFINE_MMAP_LOCK_EVENT(mmap_lock_start_locking);
 DEFINE_MMAP_LOCK_EVENT(mmap_lock_released);
 
-TRACE_EVENT_FN(mmap_lock_acquire_returned,
+TRACE_EVENT(mmap_lock_acquire_returned,
 
 	TP_PROTO(struct mm_struct *mm, const char *memcg_path, bool write,
 		bool success),
@@ -76,9 +72,7 @@ TRACE_EVENT_FN(mmap_lock_acquire_returned,
 		__get_str(memcg_path),
 		__entry->write ? "true" : "false",
 		__entry->success ? "true" : "false"
-	),
-
-	trace_mmap_lock_reg, trace_mmap_lock_unreg
+	)
 );
 
 #endif /* _TRACE_MMAP_LOCK_H */

mm/mmap_lock.c

@@ -19,43 +19,23 @@ EXPORT_TRACEPOINT_SYMBOL(mmap_lock_released);
 
 #ifdef CONFIG_MEMCG
 
-static atomic_t reg_refcount;
-
 /*
  * Size of the buffer for memcg path names. Ignoring stack trace support,
  * trace_events_hist.c uses MAX_FILTER_STR_VAL for this, so we also use it.
  */
 #define MEMCG_PATH_BUF_SIZE MAX_FILTER_STR_VAL
 
-int trace_mmap_lock_reg(void)
-{
-	atomic_inc(&reg_refcount);
-	return 0;
-}
-
-void trace_mmap_lock_unreg(void)
-{
-	atomic_dec(&reg_refcount);
-}
-
-#define TRACE_MMAP_LOCK_EVENT(type, mm, ...)                    \
-	do {                                                    \
-		char buf[MEMCG_PATH_BUF_SIZE];                  \
-		get_mm_memcg_path(mm, buf, sizeof(buf));        \
-		trace_mmap_lock_##type(mm, buf, ##__VA_ARGS__); \
+#define TRACE_MMAP_LOCK_EVENT(type, mm, ...)				\
+	do {								\
+		if (trace_mmap_lock_##type##_enabled()) {		\
+			char buf[MEMCG_PATH_BUF_SIZE];                  \
+			get_mm_memcg_path(mm, buf, sizeof(buf));        \
+			trace_mmap_lock_##type(mm, buf, ##__VA_ARGS__); \
+		}							\
 	} while (0)
 
 #else /* !CONFIG_MEMCG */
 
-int trace_mmap_lock_reg(void)
-{
-	return 0;
-}
-
-void trace_mmap_lock_unreg(void)
-{
-}
-
 #define TRACE_MMAP_LOCK_EVENT(type, mm, ...)                                   \
 	trace_mmap_lock_##type(mm, "", ##__VA_ARGS__)
 
@@ -65,16 +45,13 @@ void trace_mmap_lock_unreg(void)
 #ifdef CONFIG_MEMCG
 /*
  * Write the given mm_struct's memcg path to a buffer. If the path cannot be
- * determined or the trace event is being unregistered, empty string is written.
+ * determined, empty string is written.
  */
 static void get_mm_memcg_path(struct mm_struct *mm, char *buf, size_t buflen)
 {
 	struct mem_cgroup *memcg;
 
 	buf[0] = '\0';
-	/* No need to get path if no trace event is registered. */
-	if (!atomic_read(&reg_refcount))
-		return;
 	memcg = get_mem_cgroup_from_mm(mm);
 	if (memcg == NULL)
 		return;