perf stat: Add header for interval in JSON output

It missed to print a matching header line for intervals.

Before:

  # perf stat -a -e cycles,instructions --metric-only -j -I 500
  {"unit" : "insn per cycle"}
  {"interval" : 0.500544283}{"metric-value" : "1.96"}
  ^C

After:

  # perf stat -a -e cycles,instructions --metric-only -j -I 500
  {"unit" : "sec"}{"unit" : "insn per cycle"}
  {"interval" : 0.500515681}{"metric-value" : "2.31"}
  ^C

Signed-off-by: Namhyung Kim <[email protected]>
Acked-by: Ian Rogers <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: James Clark <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Xing Zhengjun <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

1 files changed, 4 insertions, 0 deletions

diff --git a/tools/perf/util/stat-display.c b/tools/perf/util/stat-display.c
index 99bc0db8f068..2a08c66121ec 100644
--- a/tools/perf/util/stat-display.c
+++ b/tools/perf/util/stat-display.c
@@ -858,6 +858,10 @@ static void print_metric_headers(struct perf_stat_config *config,
 		if (!config->iostat_run)
 			fputs(aggr_header_csv[config->aggr_mode], config->output);
 	}
+	if (config->json_output) {
+		if (config->interval)
+			fputs("{\"unit\" : \"sec\"}", config->output);
+	}
 	if (config->iostat_run)
 		iostat_print_header_prefix(config);