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

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Date:   Tue, 21 Apr 2020 10:53:29 -0300
From:   Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
To:     Jin Yao <yao.jin@...ux.intel.com>
Cc:     jolsa@...nel.org, peterz@...radead.org, mingo@...hat.com,
        alexander.shishkin@...ux.intel.com, Linux-kernel@...r.kernel.org,
        ak@...ux.intel.com, kan.liang@...el.com, yao.jin@...el.com
Subject: Re: [PATCH v2] perf stat: Improve runtime stat for interval mode

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@...-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@...-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?

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
 
> 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@...-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@...-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@...ux.intel.com>
> ---
>  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