perf bench: Flush stdout before starting bench suite

perf bench prints header message for bench suite before starting the
benchmark.  However if the stdout is redirected to a file and bench
suite forks child processes this (and possibly other debugging
messages too) will be repeated multiple times.

  $ perf bench sched messaging
  # Running sched/messaging benchmark...
  # 20 sender and receiver processes per group
  # 10 groups == 400 processes run

       Total time: 0.100 [sec]

  $ perf bench sched messaging > result.txt
  $ wc -l result.txt
  391

In this file, there were so many "Running sched/messaging benchmark..."
lines.  This was because stdout is converted to fully-buffered due to
the redirection and inherited child processes.  Other lines are printed
after reaping all those tasks.

So fix it by flushing stdout before starting bench suites.
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Hitoshi Mitake <h.mitake@gmail.com>
Cc: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1357637966-8216-1-git-send-email-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-bench.c

@@ -159,6 +159,7 @@ static void all_suite(struct bench_subsys *subsys)	  /* FROM HERE */
 		printf("# Running %s/%s benchmark...\n",
 		       subsys->name,
 		       suites[i].name);
+		fflush(stdout);
 
 		argv[1] = suites[i].name;
 		suites[i].fn(1, argv, NULL);
@@ -225,6 +226,7 @@ int cmd_bench(int argc, const char **argv, const char *prefix __maybe_unused)
 				printf("# Running %s/%s benchmark...\n",
 				       subsystems[i].name,
 				       subsystems[i].suites[j].name);
+			fflush(stdout);
 			status = subsystems[i].suites[j].fn(argc - 1,
 							    argv + 1, prefix);
 			goto end;