ftrace: Have funcgraph-args take affect during tracing

Currently, when function_graph is started, it looks at the option
funcgraph-args, and if it is set, it will enable tracing of the arguments.

But if tracing is already running, and the user enables funcgraph-args, it
will have no effect. Instead, it should enable argument tracing when it is
enabled, even if it means disabling the function graph tracing for a short
time in order to do the transition.

Cc: Mark Rutland <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Sven Schnelle <[email protected]>
Cc: Paul Walmsley <[email protected]>
Cc: Palmer Dabbelt <[email protected]>
Cc: Albert Ou <[email protected]>
Cc: Guo Ren <[email protected]>
Cc: Donglin Peng <[email protected]>
Cc: Zheng Yejian <[email protected]>
Link: https://lore.kernel.org/[email protected]
Acked-by: Masami Hiramatsu (Google) <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

kernel/trace/trace_functions_graph.c

@@ -464,7 +464,7 @@ static int graph_trace_init(struct trace_array *tr)
 	else
 		tr->gops->retfunc = trace_graph_return;
 
-	/* Make gops functions are visible before we start tracing */
+	/* Make gops functions visible before we start tracing */
 	smp_mb();
 
 	ret = register_ftrace_graph(tr->gops);
@@ -475,6 +475,28 @@ static int graph_trace_init(struct trace_array *tr)
 	return 0;
 }
 
+static int ftrace_graph_trace_args(struct trace_array *tr, int set)
+{
+	trace_func_graph_ent_t entry;
+
+	if (set)
+		entry = trace_graph_entry_args;
+	else
+		entry = trace_graph_entry;
+
+	/* See if there's any changes */
+	if (tr->gops->entryfunc == entry)
+		return 0;
+
+	unregister_ftrace_graph(tr->gops);
+
+	tr->gops->entryfunc = entry;
+
+	/* Make gops functions visible before we start tracing */
+	smp_mb();
+	return register_ftrace_graph(tr->gops);
+}
+
 static void graph_trace_reset(struct trace_array *tr)
 {
 	tracing_stop_cmdline_record();
@@ -1607,6 +1629,9 @@ func_graph_set_flag(struct trace_array *tr, u32 old_flags, u32 bit, int set)
 	if (bit == TRACE_GRAPH_GRAPH_TIME)
 		ftrace_graph_graph_time_control(set);
 
+	if (bit == TRACE_GRAPH_ARGS)
+		return ftrace_graph_trace_args(tr, set);
+
 	return 0;
 }