perf stat: Add --sync/-S option

--sync will tell perf stat to run sync() before starting a command.

This allows IO-heavy tests to be used with --repeat, without one
iteration impacting the other.

Elapsed time will stabilize for example:

  before:        3.971525714  seconds time elapsed  ( +-  8.56% )
  after:         3.211098537  seconds time elapsed  ( +-  1.52% )

So measurements will be more accurate.

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/n/tip-7y40wib8n1eqio7hjpn1dsrm@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 7 insertions, 0 deletions

diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index 003caa857a44..5658a770dbd7 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -128,6 +128,7 @@ static pid_t			target_tid			= -1;
 static pid_t                    child_pid                       = -1;
 static bool                     null_run                        =  false;
 static bool                     detailed_run                    =  false;
+static bool                     sync_run                        =  false;
 static bool                     big_num                         =  true;
 static int                      big_num_opt                     =  -1;
 static const char               *cpu_list;
@@ -819,6 +820,8 @@ static const struct option options[] = {
                     "null run - dont start any counters"),
         OPT_BOOLEAN('d', "detailed", &detailed_run,
                     "detailed run - start a lot of events"),
+        OPT_BOOLEAN('S', "sync", &sync_run,
+                    "call sync() before starting a run"),
         OPT_CALLBACK_NOOPT('B', "big-num", NULL, NULL, 
                            "print large numbers with thousands\' separators",
                            stat__set_big_num),
@@ -944,6 +947,10 @@ int cmd_stat(int argc, const char **argv, const char *prefix __used)
         for (run_idx = 0; run_idx < run_count; run_idx++) {
                 if (run_count != 1 && verbose)
                         fprintf(stderr, "[ perf stat: executing run #%d ... ]\n", run_idx + 1);
+
+                if (sync_run)
+                        sync();
+
                 status = run_perf_stat(argc, argv);
         }