[PATCH v2] perf stat: Improve runtime stat for interval mode

[Jin Yao]

For interval mode, the metric is printed after # if it exists. But
it's not calculated by the counts generated in this interval. See
following examples,

 root@kbl-ppc:~# perf stat -M CPI -I1000 --interval-count 2
 #           time             counts unit events
      1.000422803            764,809      inst_retired.any          #      2.9 CPI
      1.000422803          2,234,932      cycles
      2.001464585          1,960,061      inst_retired.any          #      1.6 CPI
      2.001464585          4,022,591      cycles

The second CPI should not be 1.6 (4,022,591/1,960,061 is 2.1)

 root@kbl-ppc:~# perf stat -e cycles,instructions -I1000 --interval-count 2
 #           time             counts unit events
      1.000429493          2,869,311      cycles
      1.000429493            816,875      instructions              #    0.28  insn per cycle
      2.001516426          9,260,973      cycles
      2.001516426          5,250,634      instructions              #    0.87  insn per cycle

The second 'insn per cycle' should not be 0.87 (5,250,634/9,260,973 is 0.57).

The current code uses a global variable rt_stat for tracking and
updating the std dev of runtime stat. Unlike the counts, rt_stat is
not reset for interval. While the counts are reset for interval.

perf_stat_process_counter()
{
        if (config->interval)
                init_stats(ps->res_stats);
}

So for interval, the rt_stat should be reset either.

This patch resets rt_stat before read_counters, so the runtime
stat is only calculated by the counts generated in this interval.

With this patch,

 root@kbl-ppc:~# perf stat -M CPI -I1000 --interval-count 2
 #           time             counts unit events
      1.000420924          2,408,818      inst_retired.any          #      2.1 CPI
      1.000420924          5,010,111      cycles
      2.001448579          2,798,407      inst_retired.any          #      1.6 CPI
      2.001448579          4,599,861      cycles

 root@kbl-ppc:~# perf stat -e cycles,instructions -I1000 --interval-count 2
 #           time             counts unit events
      1.000428555          2,769,714      cycles
      1.000428555            774,462      instructions              #    0.28  insn per cycle
      2.001471562          3,595,904      cycles
      2.001471562          1,243,703      instructions              #    0.35  insn per cycle

Now the second 'insn per cycle' and CPI are calculated by the counts
generated in this interval.

 v2:
 ---
 Use just existing perf_stat__reset_shadow_per_stat(&rt_stat).
 We don't need to define new function perf_stat__reset_rt_stat.

Signed-off-by: Jin Yao <yao.jin@xxxxxxxxxxxxxxx>
---
 tools/perf/Documentation/perf-stat.txt | 2 ++
 tools/perf/builtin-stat.c              | 1 +
 2 files changed, 3 insertions(+)

diff --git a/tools/perf/Documentation/perf-stat.txt b/tools/perf/Documentation/perf-stat.txt
index 4d56586b2fb9..3fb5028aef08 100644
--- a/tools/perf/Documentation/perf-stat.txt
+++ b/tools/perf/Documentation/perf-stat.txt
@@ -176,6 +176,8 @@ Print count deltas every N milliseconds (minimum: 1ms)
 The overhead percentage could be high in some cases, for instance with small, sub 100ms intervals.  Use with caution.
 	example: 'perf stat -I 1000 -e cycles -a sleep 5'
 
+If the metric exists, it is calculated by the counts generated in this interval and the metric is printed after #.
+
 --interval-count times::
 Print count deltas for fixed number of times.
 This option should be used together with "-I" option.
diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index 9207b6c45475..3f050d85c277 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -359,6 +359,7 @@ static void process_interval(void)
 	clock_gettime(CLOCK_MONOTONIC, &ts);
 	diff_timespec(&rs, &ts, &ref_time);
 
+	perf_stat__reset_shadow_per_stat(&rt_stat);
 	read_counters(&rs);
 
 	if (STAT_RECORD) {
-- 
2.17.1