function_graph: Make fgraph_update_pid_func() a stub for !DYNAMIC_FTRACE

When CONFIG_DYNAMIC_FTRACE is not set, the function
fgraph_update_pid_func() doesn't do anything. Currently, most of its logic
is within a "#ifdef CONFIG_DYNAMIC_FTRACE" block, but its variables were
declared outside that, and when DYNAMIC_FTRACE is not set, it produces
unused variable warnings.

Instead, just place it (and the helper function fgraph_pid_func()) within
the #ifdef block and have the header file use a empty stub function for
when DYNAMIC_FTRACE is not defined.

Link: https://lore.kernel.org/linux-trace-kernel/20240607094833.6a787d73@rorschach.local.home

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202406071806.BRjaC5FF-lkp@intel.com/Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/fgraph.c

@@ -1138,6 +1138,7 @@ void ftrace_graph_exit_task(struct task_struct *t)
 	kfree(ret_stack);
 }
 
+#ifdef CONFIG_DYNAMIC_FTRACE
 static int fgraph_pid_func(struct ftrace_graph_ent *trace,
 			   struct fgraph_ops *gops)
 {
@@ -1164,7 +1165,6 @@ void fgraph_update_pid_func(void)
 	if (!(graph_ops.flags & FTRACE_OPS_FL_INITIALIZED))
 		return;
 
-#ifdef CONFIG_DYNAMIC_FTRACE
 	list_for_each_entry(op, &graph_ops.subop_list, list) {
 		if (op->flags & FTRACE_OPS_FL_PID) {
 			gops = container_of(op, struct fgraph_ops, ops);
@@ -1174,8 +1174,8 @@ void fgraph_update_pid_func(void)
 				static_call_update(fgraph_func, gops->entryfunc);
 		}
 	}
-#endif
 }
+#endif
 
 /* Allocate a return stack for each task */
 static int start_graph_tracing(void)

kernel/trace/ftrace_internal.h

@@ -52,7 +52,11 @@ static inline int ftrace_shutdown_subops(struct ftrace_ops *ops, struct ftrace_o
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 extern int ftrace_graph_active;
+# ifdef CONFIG_DYNAMIC_FTRACE
 extern void fgraph_update_pid_func(void);
+# else
+static inline void fgraph_update_pid_func(void) {}
+# endif
 #else /* !CONFIG_FUNCTION_GRAPH_TRACER */
 # define ftrace_graph_active 0
 static inline void fgraph_update_pid_func(void) {}