perf stat: Fix JSON output in metric-only mode

It generated a broken JSON output when aggregation mode or cgroup is
used with --metric-only option.  Also get rid of the header line and
make the output single line for each entry.

It needs to know whether the current metric is the first one or not.
So add 'first' field in the outstate and mark it false after printing.

Before:

  # perf stat -a -j --metric-only true
  {"unit" : "GHz"}{"unit" : "insn per cycle"}{"unit" : "branch-misses of all branches"}
  {{"metric-value" : "0.797"}{"metric-value" : "1.65"}{"metric-value" : "0.89"}
   ^

  # perf stat -a -j --metric-only --per-socket true
  {"unit" : "GHz"}{"unit" : "insn per cycle"}{"unit" : "branch-misses of all branches"}
  {"socket" : "S0", "aggregate-number" : 8, {"metric-value" : "0.295"}{"metric-value" : "1.88"}{"metric-value" : "0.64"}
                                           ^

After:

  # perf stat -a -j --metric-only true
  {"GHz" : "0.990", "insn per cycle" : "2.06", "branch-misses of all branches" : "0.59"}

  # perf stat -a -j --metric-only --per-socket true
  {"socket" : "S0", "aggregate-number" : 8, "GHz" : "0.439", "insn per cycle" : "2.14", "branch-misses of all branches" : "0.51"}
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Athira Jajeev <atrajeev@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Link: https://lore.kernel.org/r/20221123180208.2068936-14-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/stat-display.c

@@ -279,9 +279,6 @@ static void print_aggr_id_json(struct perf_stat_config *config,
 {
 	FILE *output = config->output;
 
-	if (!config->interval)
-		fputc('{', output);
-
 	switch (config->aggr_mode) {
 	case AGGR_CORE:
 		fprintf(output, "\"core\" : \"S%d-D%d-C%d\", \"aggregate-number\" : %d, ",
@@ -335,6 +332,7 @@ static void aggr_printout(struct perf_stat_config *config,
 struct outstate {
 	FILE *fh;
 	bool newline;
+	bool first;
 	const char *prefix;
 	int  nfields;
 	int  nr;
@@ -491,6 +489,7 @@ static void print_metric_only(struct perf_stat_config *config,
 
 	color_snprintf(str, sizeof(str), color ?: "", fmt, val);
 	fprintf(out, "%*s ", mlen, str);
+	os->first = false;
 }
 
 static void print_metric_only_csv(struct perf_stat_config *config __maybe_unused,
@@ -512,6 +511,7 @@ static void print_metric_only_csv(struct perf_stat_config *config __maybe_unused
 		ends++;
 	*ends = 0;
 	fprintf(out, "%s%s", vals, config->csv_sep);
+	os->first = false;
 }
 
 static void print_metric_only_json(struct perf_stat_config *config __maybe_unused,
@@ -532,7 +532,8 @@ static void print_metric_only_json(struct perf_stat_config *config __maybe_unuse
 	while (isdigit(*ends) || *ends == '.')
 		ends++;
 	*ends = 0;
-	fprintf(out, "{\"metric-value\" : \"%s\"}", vals);
+	fprintf(out, "%s\"%s\" : \"%s\"", os->first ? "" : ", ", unit, vals);
+	os->first = false;
 }
 
 static void new_line_metric(struct perf_stat_config *config __maybe_unused,
@@ -561,7 +562,7 @@ static void print_metric_header(struct perf_stat_config *config,
 	unit = fixunit(tbuf, os->evsel, unit);
 
 	if (config->json_output)
-		fprintf(os->fh, "{\"unit\" : \"%s\"}", unit);
+		return;
 	else if (config->csv_output)
 		fprintf(os->fh, "%s%s", unit, config->csv_sep);
 	else
@@ -821,6 +822,8 @@ static void print_counter_aggrdata(struct perf_stat_config *config,
 	run = aggr->counts.run;
 
 	if (!metric_only) {
+		if (config->json_output)
+			fputc('{', output);
 		if (os->prefix)
 			fprintf(output, "%s", os->prefix);
 		else if (config->summary && config->csv_output &&
@@ -844,9 +847,12 @@ static void print_metric_begin(struct perf_stat_config *config,
 	struct aggr_cpu_id id;
 	struct evsel *evsel;
 
+	os->first = true;
 	if (!config->metric_only)
 		return;
 
+	if (config->json_output)
+		fputc('{', config->output);
 	if (os->prefix)
 		fprintf(config->output, "%s", os->prefix);
 
@@ -855,7 +861,7 @@ static void print_metric_begin(struct perf_stat_config *config,
 	aggr = &evsel->stats->aggr[aggr_idx];
 	aggr_printout(config, evsel, id, aggr->nr);
 
-	print_cgroup(config, os->cgrp);
+	print_cgroup(config, os->cgrp ? : evsel->cgrp);
 }
 
 static void print_metric_end(struct perf_stat_config *config)
@@ -863,6 +869,8 @@ static void print_metric_end(struct perf_stat_config *config)
 	if (!config->metric_only)
 		return;
 
+	if (config->json_output)
+		fputc('}', config->output);
 	fputc('\n', config->output);
 }
 
@@ -1005,11 +1013,9 @@ static void print_metric_headers_csv(struct perf_stat_config *config,
 		fputs(aggr_header_csv[config->aggr_mode], config->output);
 }
 
-static void print_metric_headers_json(struct perf_stat_config *config,
+static void print_metric_headers_json(struct perf_stat_config *config __maybe_unused,
 				      bool no_indent __maybe_unused)
 {
-	if (config->interval)
-		fputs("{\"unit\" : \"sec\"}", config->output);
 }
 
 static void print_metric_headers(struct perf_stat_config *config,
@@ -1049,7 +1055,9 @@ static void print_metric_headers(struct perf_stat_config *config,
 					      &config->metric_events,
 					      &rt_stat);
 	}
-	fputc('\n', config->output);
+
+	if (!config->json_output)
+		fputc('\n', config->output);
 }
 
 static void prepare_interval(struct perf_stat_config *config,
@@ -1058,17 +1066,14 @@ static void prepare_interval(struct perf_stat_config *config,
 	if (config->iostat_run)
 		return;
 
-	if (config->csv_output)
+	if (config->json_output)
+		scnprintf(prefix, len, "\"interval\" : %lu.%09lu, ",
+			  (unsigned long) ts->tv_sec, ts->tv_nsec);
+	else if (config->csv_output)
 		scnprintf(prefix, len, "%lu.%09lu%s",
 			  (unsigned long) ts->tv_sec, ts->tv_nsec, config->csv_sep);
-	else if (!config->json_output)
-		scnprintf(prefix, len, "%6lu.%09lu ",
-			  (unsigned long) ts->tv_sec, ts->tv_nsec);
-	else if (!config->metric_only)
-		scnprintf(prefix, len, "{\"interval\" : %lu.%09lu, ",
-			  (unsigned long) ts->tv_sec, ts->tv_nsec);
 	else
-		scnprintf(prefix, len, "{\"interval\" : %lu.%09lu}",
+		scnprintf(prefix, len, "%6lu.%09lu ",
 			  (unsigned long) ts->tv_sec, ts->tv_nsec);
 }
 
@@ -1365,6 +1370,7 @@ void evlist__print_counters(struct evlist *evlist, struct perf_stat_config *conf
 	char buf[64];
 	struct outstate os = {
 		.fh = config->output,
+		.first = true,
 	};
 
 	if (config->iostat_run)