perf ftrace: Detect whether ftrace is enabled on system

To make error messages more accurate, this change detects whether ftrace is
enabled on system by checking trace file "set_ftrace_pid".

Before:

  # perf ftrace
  failed to reset ftrace
  #

After:

  # perf ftrace
  ftrace is not supported on this system
  #

Committer testing:

Doing it in an unprivileged toolbox container on Fedora 40:

Before:

  acme@number:~/git/perf-tools-next$ toolbox enter perf
  ⬢[acme@toolbox perf-tools-next]$ sudo su -
  ⬢[root@toolbox ~]# ~acme/bin/perf ftrace
  failed to reset ftrace
  ⬢[root@toolbox ~]#

After this patch:

  ⬢[root@toolbox ~]# ~acme/bin/perf ftrace
  ftrace is not supported on this system
  ⬢[root@toolbox ~]#

Maybe we could check if we are in such as situation, inside an
unprivileged container, and provide a HINT line?

Reviewed-by: James Clark <[email protected]>
Signed-off-by: Changbin Du <[email protected]>
Tested-by: Arnaldo Carvalho de Melo <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Changbin Du <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: Changbin Du

tools/perf/builtin-ftrace.c

@@ -80,6 +80,24 @@ static bool check_ftrace_capable(void)
 	return false;
 }
 
+static bool is_ftrace_supported(void)
+{
+	char *file;
+	bool supported = false;
+
+	file = get_tracing_file("set_ftrace_pid");
+	if (!file) {
+		pr_debug("cannot get tracing file set_ftrace_pid\n");
+		return false;
+	}
+
+	if (!access(file, F_OK))
+		supported = true;
+
+	put_tracing_file(file);
+	return supported;
+}
+
 static int __write_tracing_file(const char *name, const char *val, bool append)
 {
 	char *file;
@@ -1583,6 +1601,11 @@ int cmd_ftrace(int argc, const char **argv)
 	if (!check_ftrace_capable())
 		return -1;
 
+	if (!is_ftrace_supported()) {
+		pr_err("ftrace is not supported on this system\n");
+		return -ENOTSUP;
+	}
+
 	ret = perf_config(perf_ftrace_config, &ftrace);
 	if (ret < 0)
 		return -1;