perf ftrace: Add option --tid to filter by thread id

This allows us to trace single thread instead of the whole process.
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/20200808023141.14227-17-changbin.du@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-ftrace.txt

@@ -39,6 +39,9 @@ OPTIONS
 --pid=::
 	Trace on existing process id (comma separated list).
 
+--tid=::
+	Trace on existing thread id (comma separated list).
+
 -D::
 --delay::
 	Time (ms) to wait before starting tracing after program start.

tools/perf/builtin-ftrace.c

@@ -812,6 +812,9 @@ int cmd_ftrace(int argc, const char **argv)
 		    "Show available functions to filter"),
 	OPT_STRING('p', "pid", &ftrace.target.pid, "pid",
 		   "trace on existing process id"),
+	/* TODO: Add short option -t after -t/--tracer can be removed. */
+	OPT_STRING(0, "tid", &ftrace.target.tid, "tid",
+		   "trace on existing thread id (exclusive to --pid)"),
 	OPT_INCR('v', "verbose", &verbose,
 		 "be more verbose"),
 	OPT_BOOLEAN('a', "all-cpus", &ftrace.target.system_wide,