perf stat: Fix interval output values

We broke interval data displays with commit:

  3f416f22d1 ("perf stat: Do not clean event's private stats")

This commit removed stats cleaning, which is important for '-r' option
to carry counters data over the whole run. But it's necessary to clean
it for interval mode, otherwise the displayed value is avg of all
previous values.

Before:
  $ perf stat -e cycles -a -I 1000 record
  #           time             counts unit events
       1.000240796         75,216,287      cycles
       2.000512791        107,823,524      cycles

  $ perf stat report
  #           time             counts unit events
       1.000240796         75,216,287      cycles
       2.000512791         91,519,906      cycles

Now:
  $ perf stat report
  #           time             counts unit events
       1.000240796         75,216,287      cycles
       2.000512791        107,823,524      cycles

Notice the second value being bigger (91,.. < 107,..).

This could be easily verified by using perf script which displays raw
stat data:

  $ perf script
  CPU  THREAD       VAL         ENA         RUN        TIME EVENT
    0      -1  23855779  1000209530  1000209530  1000240796 cycles
    1      -1  33340397  1000224964  1000224964  1000240796 cycles
    2      -1  15835415  1000226695  1000226695  1000240796 cycles
    3      -1   2184696  1000228245  1000228245  1000240796 cycles
    0      -1  97014312  2000514533  2000514533  2000512791 cycles
    1      -1  46121497  2000543795  2000543795  2000512791 cycles
    2      -1  32269530  2000543566  2000543566  2000512791 cycles
    3      -1   7634472  2000544108  2000544108  2000512791 cycles

The sum of the first 4 values is the first interval aggregated value:

  23855779 + 33340397 + 15835415 + 2184696 = 75,216,287

The sum of the second 4 values minus first value is the second interval
aggregated value:

  97014312 + 46121497 + 32269530 + 7634472 - 75216287 = 107,823,524

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1454485436-20639-1-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/stat.c

@@ -311,6 +311,16 @@ int perf_stat_process_counter(struct perf_stat_config *config,
 
 	aggr->val = aggr->ena = aggr->run = 0;
 
+	/*
+	 * We calculate counter's data every interval,
+	 * and the display code shows ps->res_stats
+	 * avg value. We need to zero the stats for
+	 * interval mode, otherwise overall avg running
+	 * averages will be shown for each interval.
+	 */
+	if (config->interval)
+		init_stats(ps->res_stats);
+
 	if (counter->per_pkg)
 		zero_per_pkg(counter);