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

[Arnaldo Carvalho de Melo]

Em Wed, Apr 22, 2020 at 08:53:41AM +0800, Jin, Yao escreveu:
> Hi Arnaldo,
> 
> On 4/21/2020 9:53 PM, Arnaldo Carvalho de Melo wrote:
> > Em Mon, Apr 20, 2020 at 10:54:17PM +0800, Jin Yao escreveu:
> > > 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.
> > 
> >                                s/either/too/g right?
> > 
> 
> Yes, should use "too" here. :)
> 
> > And please try and find what was the cset that introduced the problem,
> > so that we can have a Fixes: line and the stable series can pick it, ok?
> >
> > - Arnaldo
> > 
> 
> I have tried to find the patch which introduced this issue.
> 
> 51fd2df1e882 ("perf stat: Fix interval output values").
> 
> This patch zeros stats for interval mode. I just think it should reset
> rt_stat too.

> But I really don't know if it's fair to this patch so I don't add it in my
> patch description.

I think you are right that 51fd2df1e882 patch missed the fix you're
providing now, so I think it is fair to say that your current patch
fixes 51fd2df1e882, so I added it in the Fixes tag for the patch
discussed in this message, thanks.

- Arnaldo
 
> Thanks
> Jin Yao
> 
> > > 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
> > > 
> > 

-- 

- Arnaldo