perf stat: Fix wrong per-thread runtime stat for interval mode

  root@kbl-ppc:~# perf stat --per-thread -e cycles,instructions -I1000 --interval-count 2
       1.004171683             perf-3696              8,747,311      cycles
          ...
       1.004171683             perf-3696                691,730      instructions              #    0.08  insn per cycle
          ...
       2.006490373             perf-3696              1,749,936      cycles
          ...
       2.006490373             perf-3696              1,484,582      instructions              #    0.28  insn per cycle
          ...

Let's see interval 2.006490373

  perf-3696              1,749,936      cycles
  perf-3696              1,484,582      instructions              #    0.28  insn per cycle

insn per cycle = 1,484,582 / 1,749,936 = 0.85.

But now it's 0.28, that's not correct.

stat_config.stats[] records the per-thread runtime stat. But for
interval mode, it should be reset for each interval.

So now, with this patch,

  root@kbl-ppc:~# perf stat --per-thread -e cycles,instructions -I1000 --interval-count 2
       1.005818121             perf-8633              9,898,045      cycles
          ...
       1.005818121             perf-8633                693,298      instructions              #    0.07  insn per cycle
          ...
       2.007863743             perf-8633              1,551,619      cycles
          ...
       2.007863743             perf-8633              1,317,514      instructions              #    0.85  insn per cycle
          ...

Let's check interval 2.007863743.

insn per cycle = 1,317,514 / 1,551,619 = 0.85. It's correct.

This patch creates runtime_stat_reset, places it next to
untime_stat_new/runtime_stat_delete and moves all runtime_stat
functions before process_interval.

Committer testing:

After the patch:

  # perf stat --per-thread -e cycles,instructions -I1000 --interval-count 2  |& grep sssd_nss-1130
     2.011309774  sssd_nss-1130   56,585  cycles
     2.011309774  sssd_nss-1130   13,121  instructions  # 0.23 insn per cycle
  # python
  >>> 13121.0 / 56585
  0.23188124061146947
  >>>

Fixes: commit 14e72a21 ("perf stat: Update or print per-thread stats")
Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
Reviewed-by: Jiri Olsa <jolsa@redhat.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jin Yao <yao.jin@intel.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20200520042737.24160-2-yao.jin@linux.intel.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>