perf bench: Use unbuffered output when pipe/tee'ing to a file

The output of 'perf bench' gets buffered when I pipe it to a file or to
tee, in such a way that I can see it only at the end.

E.g.

  $ perf bench internals synthesize -t
  < output comes out fine after each test run >

  $ perf bench internals synthesize -t | tee file.txt
  < output comes out only at the end of all tests >

This patch resolves this issue for 'bench' and 'test' subcommands.

See, also:

  $ perf bench mem all | tee file.txt
  $ perf bench sched all | tee file.txt
  $ perf bench internals all -t | tee file.txt
  $ perf bench internals all | tee file.txt

Committer testing:

It really gets staggered, i.e. outputs in bursts, when the buffer fills
up and has to be drained to make up space for more output.
Suggested-by: Riccardo Mancini <rickyman7@gmail.com>
Signed-off-by: Sohaib Mohamed <sohaib.amhmd@gmail.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Fabian Hemmer <copy@copy.sh>
Cc: Ian Rogers <irogers@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20211119061409.78004-1-sohaib.amhmd@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-bench.c

@@ -226,7 +226,6 @@ static void run_collection(struct collection *coll)
 		if (!bench->fn)
 			break;
 		printf("# Running %s/%s benchmark...\n", coll->name, bench->name);
-		fflush(stdout);
 
 		argv[1] = bench->name;
 		run_bench(coll->name, bench->name, bench->fn, 1, argv);
@@ -247,6 +246,9 @@ int cmd_bench(int argc, const char **argv)
 	struct collection *coll;
 	int ret = 0;
 
+	/* Unbuffered output */
+	setvbuf(stdout, NULL, _IONBF, 0);
+
 	if (argc < 2) {
 		/* No collection specified. */
 		print_usage();
@@ -300,7 +302,6 @@ int cmd_bench(int argc, const char **argv)
 
 			if (bench_format == BENCH_FORMAT_DEFAULT)
 				printf("# Running '%s/%s' benchmark:\n", coll->name, bench->name);
-			fflush(stdout);
 			ret = run_bench(coll->name, bench->name, bench->fn, argc-1, argv+1);
 			goto end;
 		}

tools/perf/tests/builtin-test.c

@@ -607,6 +607,9 @@ int cmd_test(int argc, const char **argv)
         if (ret < 0)
                 return ret;
 
+	/* Unbuffered output */
+	setvbuf(stdout, NULL, _IONBF, 0);
+
 	argc = parse_options_subcommand(argc, argv, test_options, test_subcommands, test_usage, 0);
 	if (argc >= 1 && !strcmp(argv[0], "list"))
 		return perf_test__list(argc - 1, argv + 1);