perf stat: Document and simplify interval timestamps

Rename 'prefix' to 'timestamp' because that's all it does, except in
iostat mode where it's slightly overloaded, but still includes a
timestamp. This reveals a problem with iostat and JSON mode so document
this.

Make it more explicit that these are printed in interval mode by
changing 'if (prefix)' to 'if (interval)' which reveals an unnecessary
'else if (... && !interval)' which can be removed.

Signed-off-by: James Clark <james.clark@linaro.org>
Tested-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Yicong Yang <yangyicong@hisilicon.com>
Link: https://lore.kernel.org/r/20241112160048.951213-5-james.clark@linaro.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/arch/x86/util/iostat.c

@@ -403,6 +403,10 @@ void iostat_prefix(struct evlist *evlist,
 	struct iio_root_port *rp = evlist->selected->priv;
 
 	if (rp) {
+		/*
+		 * TODO: This is the incorrect format in JSON mode.
+		 *       See prepare_timestamp()
+		 */
 		if (ts)
 			sprintf(prefix, "%6lu.%09lu%s%04x:%02x%s",
 				ts->tv_sec, ts->tv_nsec,

tools/perf/util/stat-display.c

@@ -118,7 +118,8 @@ struct outstate {
 	FILE *fh;
 	bool newline;
 	bool first;
-	const char *prefix;
+	/* Lines are timestamped in --interval-print mode */
+	char timestamp[64];
 	int  nfields;
 	int  aggr_nr;
 	struct aggr_cpu_id id;
@@ -419,8 +420,8 @@ static inline void __new_line_std_csv(struct perf_stat_config *config,
 				      struct outstate *os)
 {
 	fputc('\n', os->fh);
-	if (os->prefix)
-		fputs(os->prefix, os->fh);
+	if (config->interval)
+		fputs(os->timestamp, os->fh);
 	aggr_printout(config, os, os->evsel, os->id, os->aggr_nr);
 }
 
@@ -523,8 +524,8 @@ static void new_line_json(struct perf_stat_config *config, void *ctx)
 
 	fputs("\n{", os->fh);
 	os->first = true;
-	if (os->prefix)
-		json_out(os, "%s", os->prefix);
+	if (config->interval)
+		json_out(os, "%s", os->timestamp);
 
 	aggr_printout(config, os, os->evsel, os->id, os->aggr_nr);
 }
@@ -1091,13 +1092,13 @@ static void print_counter_aggrdata(struct perf_stat_config *config,
 			os->first = true;
 			fputc('{', output);
 		}
-		if (os->prefix) {
+		if (config->interval) {
 			if (config->json_output)
-				json_out(os, "%s", os->prefix);
+				json_out(os, "%s", os->timestamp);
 			else
-				fprintf(output, "%s", os->prefix);
+				fprintf(output, "%s", os->timestamp);
 		} else if (config->summary && config->csv_output &&
-			   !config->no_csv_summary && !config->interval)
+			   !config->no_csv_summary)
 			fprintf(output, "%s%s", "summary", config->csv_sep);
 	}
 
@@ -1124,11 +1125,11 @@ static void print_metric_begin(struct perf_stat_config *config,
 	if (config->json_output)
 		fputc('{', config->output);
 
-	if (os->prefix) {
+	if (config->interval) {
 		if (config->json_output)
-			json_out(os, "%s", os->prefix);
+			json_out(os, "%s", os->timestamp);
 		else
-			fprintf(config->output, "%s", os->prefix);
+			fprintf(config->output, "%s", os->timestamp);
 	}
 	evsel = evlist__first(evlist);
 	id = config->aggr_map->map[aggr_idx];
@@ -1349,20 +1350,20 @@ static void print_metric_headers(struct perf_stat_config *config,
 		fputc('\n', config->output);
 }
 
-static void prepare_interval(struct perf_stat_config *config,
-			     char *prefix, size_t len, struct timespec *ts)
+static void prepare_timestamp(struct perf_stat_config *config,
+			      struct outstate *os, struct timespec *ts)
 {
 	if (config->iostat_run)
 		return;
 
 	if (config->json_output)
-		scnprintf(prefix, len, "\"interval\" : %lu.%09lu",
+		scnprintf(os->timestamp, sizeof(os->timestamp), "\"interval\" : %lu.%09lu",
 			  (unsigned long) ts->tv_sec, ts->tv_nsec);
 	else if (config->csv_output)
-		scnprintf(prefix, len, "%lu.%09lu%s",
+		scnprintf(os->timestamp, sizeof(os->timestamp), "%lu.%09lu%s",
 			  (unsigned long) ts->tv_sec, ts->tv_nsec, config->csv_sep);
 	else
-		scnprintf(prefix, len, "%6lu.%09lu ",
+		scnprintf(os->timestamp, sizeof(os->timestamp), "%6lu.%09lu ",
 			  (unsigned long) ts->tv_sec, ts->tv_nsec);
 }
 
@@ -1685,9 +1686,7 @@ void evlist__print_counters(struct evlist *evlist, struct perf_stat_config *conf
 			    int argc, const char **argv)
 {
 	bool metric_only = config->metric_only;
-	int interval = config->interval;
 	struct evsel *counter;
-	char buf[64];
 	struct outstate os = {
 		.fh = config->output,
 		.first = true,
@@ -1698,10 +1697,8 @@ void evlist__print_counters(struct evlist *evlist, struct perf_stat_config *conf
 	if (config->iostat_run)
 		evlist->selected = evlist__first(evlist);
 
-	if (interval) {
-		os.prefix = buf;
-		prepare_interval(config, buf, sizeof(buf), ts);
-	}
+	if (config->interval)
+		prepare_timestamp(config, &os, ts);
 
 	print_header(config, _target, evlist, argc, argv);
 
@@ -1720,7 +1717,7 @@ void evlist__print_counters(struct evlist *evlist, struct perf_stat_config *conf
 	case AGGR_THREAD:
 	case AGGR_GLOBAL:
 		if (config->iostat_run) {
-			iostat_print_counters(evlist, config, ts, buf,
+			iostat_print_counters(evlist, config, ts, os.timestamp,
 					      (iostat_print_counter_t)print_counter, &os);
 		} else if (config->cgroup_list) {
 			print_cgroup_counter(config, evlist, &os);