perf ftrace: Flush output after each writing

The pager will result stdout in full buffering mode instead of line
buffering. We need to make the trace visible timely.

Signed-off-by: Changbin Du <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[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

@@ -650,6 +650,8 @@ static int __cmd_ftrace(struct perf_ftrace *ftrace)
 				break;
 			if (fwrite(buf, n, 1, stdout) != 1)
 				break;
+			/* flush output since stdout is in full buffering mode due to pager */
+			fflush(stdout);
 		}
 	}