linux-kernel - Re: [PATCH] perf stat: Use proper cpu for shadow stats

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Date:   Sat, 14 Nov 2020 09:23:39 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Ian Rogers <irogers@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>, Sam Xi <xyzsam@...gle.com>
Subject: Re: [PATCH] perf stat: Use proper cpu for shadow stats

On Sat, Nov 14, 2020 at 7:16 AM Jiri Olsa <jolsa@...hat.com> wrote:
>
> On Fri, Nov 13, 2020 at 02:02:36PM +0900, Namhyung Kim wrote:
> > Currently perf stat shows some metrics (like IPC) for defined events.
> > But when no aggregation mode is used (-A option), it shows incorrect
> > values since it used a value from a different cpu.
> >
> > Before:
> >
> >   $ perf stat -aA -e cycles,instructions sleep 1
> >
> >    Performance counter stats for 'system wide':
> >
> >   CPU0      116,057,380      cycles
> >   CPU1       86,084,722      cycles
> >   CPU2       99,423,125      cycles
> >   CPU3       98,272,994      cycles
> >   CPU0       53,369,217      instructions      #    0.46  insn per cycle
> >   CPU1       33,378,058      instructions      #    0.29  insn per cycle
> >   CPU2       58,150,086      instructions      #    0.50  insn per cycle
> >   CPU3       40,029,703      instructions      #    0.34  insn per cycle
> >
> >        1.001816971 seconds time elapsed
> >
> > So the IPC for CPU1 should be 0.38 (= 33,378,058 / 86,084,722)
> > but it was 0.29 (= 33,378,058 / 116,057,380) and so on.
> >
> > After:
> >
> >   $ perf stat -aA -e cycles,instructions sleep 1
> >
> >    Performance counter stats for 'system wide':
> >
> >   CPU0      109,621,384      cycles
> >   CPU1      159,026,454      cycles
> >   CPU2       99,460,366      cycles
> >   CPU3      124,144,142      cycles
> >   CPU0       44,396,706      instructions      #    0.41  insn per cycle
> >   CPU1      120,195,425      instructions      #    0.76  insn per cycle
> >   CPU2       44,763,978      instructions      #    0.45  insn per cycle
> >   CPU3       69,049,079      instructions      #    0.56  insn per cycle
> >
> >        1.001910444 seconds time elapsed
> >
> > Reported-by: Sam Xi <xyzsam@...gle.com>
> > Fixes: 44d49a600259 ("perf stat: Support metrics in --per-core/socket mode")
> > Signed-off-by: Namhyung Kim <namhyung@...nel.org>
>
> nice catch! would be great to have test for this
>
> Acked-by: Jiri Olsa <jolsa@...hat.com>

Thanks!  Will add a test later.

Thanks,
Namhyung