perf ftrace: Add option -D/--delay to delay tracing

This adds an option '-D/--delay' to allow us to start tracing some times
later after workload is launched.
Signed-off-by: Changbin Du <changbin.du@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: http://lore.kernel.org/lkml/20200808023141.14227-16-changbin.du@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-ftrace.txt

@@ -39,6 +39,10 @@ OPTIONS
 --pid=::
 	Trace on existing process id (comma separated list).
 
+-D::
+--delay::
+	Time (ms) to wait before starting tracing after program start.
+
 -a::
 --all-cpus::
 	Force system-wide collection.  Scripts run without a <command>

tools/perf/builtin-ftrace.c

@@ -49,6 +49,7 @@ struct perf_ftrace {
 	int			graph_noirqs;
 	int			graph_verbose;
 	int			graph_thresh;
+	unsigned int		initial_delay;
 };
 
 struct filter_entry {
@@ -596,13 +597,23 @@ static int __cmd_ftrace(struct perf_ftrace *ftrace, int argc, const char **argv)
 	/* display column headers */
 	read_tracing_file_to_stdout("trace");
 
+	if (!ftrace->initial_delay) {
 		if (write_tracing_file("tracing_on", "1") < 0) {
 			pr_err("can't enable tracing\n");
 			goto out_close_fd;
 		}
+	}
 
 	perf_evlist__start_workload(ftrace->evlist);
 
+	if (ftrace->initial_delay) {
+		usleep(ftrace->initial_delay * 1000);
+		if (write_tracing_file("tracing_on", "1") < 0) {
+			pr_err("can't enable tracing\n");
+			goto out_close_fd;
+		}
+	}
+
 	while (!done) {
 		if (poll(&pollfd, 1, -1) < 0)
 			break;
@@ -827,6 +838,8 @@ int cmd_ftrace(int argc, const char **argv)
 		     "size of per cpu buffer", parse_buffer_size),
 	OPT_BOOLEAN(0, "inherit", &ftrace.inherit,
 		    "trace children processes"),
+	OPT_UINTEGER('D', "delay", &ftrace.initial_delay,
+		     "ms to wait before starting tracing after program start"),
 	OPT_END()
 	};