perf stat: Add support for --initial-delay option

When measuring workloads the startup phase -- doing page faults, dynamic
linking, opening files -- is often very different from the rest of the
workload.  Especially with smaller kernels and using counter
multiplexing this can give significant measurement errors.

Multiplexing assumes that the workload is mostly the same over longer
periods. But at startup there is typically some spike of activity which
is relatively short.  If many groups are multiplexing the one group
seeing the spike, and which is then scaled up over the time to run all
groups, may see a significant error.

Also in general it's often not useful to measure the startup, because it
is so different from the rest.

One way around this is to use interval mode and discard the first
sample, but this can be awkward because interval mode doesn't support
intervals of less than 100ms, and also a useful interval is not
necessarily the same as a useful startup delay.

This patch adds a new --initial-delay / -D option to skip measuring for
the startup phase. The time can be specified in ms

Here's a simple example:

perf stat -e page-faults bash -c 'for i in $(seq 100000) ; do true ; done'
...
             3,721 page-faults
...

If we just wait 20 ms the number of page faults is 1/3 less:

perf stat -D 20 -e page-faults bash -c 'for i in $(seq 100000) ; do true ; done'
...
             2,823 page-faults
...

So we filtered out most of the startup noise from bash.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Jiri Olsa <jolsa@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1375490473-1503-4-git-send-email-andi@firstfloor.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-stat.txt

@@ -132,6 +132,11 @@ is a useful mode to detect imbalance between physical cores.  To enable this mod
 use --per-core in addition to -a. (system-wide).  The output includes the
 core number and the number of online logical processors on that physical processor.
 
+-D msecs::
+--initial-delay msecs::
+After starting the program, wait msecs before measuring. This is useful to
+filter out the startup phase of the program, which is often very different.
+
 EXAMPLES
 --------
 

tools/perf/builtin-stat.c

@@ -100,6 +100,7 @@ static const char		*pre_cmd			= NULL;
 static const char		*post_cmd			= NULL;
 static bool			sync_run			= false;
 static unsigned int		interval			= 0;
+static unsigned int		initial_delay			= 0;
 static bool			forever				= false;
 static struct timespec		ref_time;
 static struct cpu_map		*aggr_map;
@@ -254,7 +255,8 @@ static int create_perf_stat_counter(struct perf_evsel *evsel)
 	if (!perf_target__has_task(&target) &&
 	    perf_evsel__is_group_leader(evsel)) {
 		attr->disabled = 1;
-		attr->enable_on_exec = 1;
+		if (!initial_delay)
+			attr->enable_on_exec = 1;
 	}
 
 	return perf_evsel__open_per_thread(evsel, evsel_list->threads);
@@ -416,6 +418,20 @@ static void print_interval(void)
 	}
 }
 
+static void handle_initial_delay(void)
+{
+	struct perf_evsel *counter;
+
+	if (initial_delay) {
+		const int ncpus = cpu_map__nr(evsel_list->cpus),
+			nthreads = thread_map__nr(evsel_list->threads);
+
+		usleep(initial_delay * 1000);
+		list_for_each_entry(counter, &evsel_list->entries, node)
+			perf_evsel__enable(counter, ncpus, nthreads);
+	}
+}
+
 static int __run_perf_stat(int argc, const char **argv)
 {
 	char msg[512];
@@ -486,6 +502,7 @@ static int __run_perf_stat(int argc, const char **argv)
 
 	if (forks) {
 		perf_evlist__start_workload(evsel_list);
+		handle_initial_delay();
 
 		if (interval) {
 			while (!waitpid(child_pid, &status, WNOHANG)) {
@@ -497,6 +514,7 @@ static int __run_perf_stat(int argc, const char **argv)
 		if (WIFSIGNALED(status))
 			psignal(WTERMSIG(status), argv[0]);
 	} else {
+		handle_initial_delay();
 		while (!done) {
 			nanosleep(&ts, NULL);
 			if (interval)
@@ -1419,6 +1437,8 @@ int cmd_stat(int argc, const char **argv, const char *prefix __maybe_unused)
 		     "aggregate counts per processor socket", AGGR_SOCKET),
 	OPT_SET_UINT(0, "per-core", &aggr_mode,
 		     "aggregate counts per physical processor core", AGGR_CORE),
+	OPT_UINTEGER('D', "delay", &initial_delay,
+		     "ms to wait before starting measurement after program start"),
 	OPT_END()
 	};
 	const char * const stat_usage[] = {