linux-kernel - Re: [PATCH v2 1/2] libperf: Add processing to scale the counters obtained during the read() system call when multiplexing

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Message-ID: <CAL_JsqLrHLmEM06mxJ8V304VTmKHYU8Pca5h=jMJQe1beoXbDw@mail.gmail.com>
Date:   Tue, 5 Oct 2021 11:36:18 -0500
From:   Rob Herring <robh@...nel.org>
To:     "nakamura.shun@...itsu.com" <nakamura.shun@...itsu.com>
Cc:     Jiri Olsa <jolsa@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "acme@...nel.org" <acme@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "irogers@...gle.com" <irogers@...gle.com>,
        "linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] libperf: Add processing to scale the counters
 obtained during the read() system call when multiplexing

On Tue, Sep 28, 2021 at 7:41 AM nakamura.shun@...itsu.com
<nakamura.shun@...itsu.com> wrote:
>
> Hi Jirka
>
> > On Wed, Sep 22, 2021 at 07:16:26PM +0900, Shunsuke Nakamura wrote:
> > > From: nakamura shunsuke <nakamura.shun@...itsu.com>
> > >
> > > perf_evsel__read() scales counters obtained by RDPMC during multiplexing, but
> > > does not scale counters obtained by read() system call.
> > >
> > > Add processing to perf_evsel__read() to scale the counters obtained during the
> > > read() system call when multiplexing.
> > >
> > >
> > > Signed-off-by: Shunsuke Nakamura <nakamura.shun@...itsu.com>
> > > ---
> > >  tools/lib/perf/evsel.c | 6 ++++++
> > >  1 file changed, 6 insertions(+)
> > >
> > > diff --git a/tools/lib/perf/evsel.c b/tools/lib/perf/evsel.c
> > > index 8441e3e1aaac..0ebd1d34436f 100644
> > > --- a/tools/lib/perf/evsel.c
> > +++ b/tools/lib/perf/evsel.c
> > > @@ -18,6 +18,7 @@
> > >  #include <sys/ioctl.h>
> > >  #include <sys/mman.h>
> > >  #include <asm/bug.h>
> > > +#include <linux/math64.h>
> > >
> > >  void perf_evsel__init(struct perf_evsel *evsel, struct perf_event_attr *attr,
> > >                      int idx)
> > > @@ -321,6 +322,11 @@ int perf_evsel__read(struct perf_evsel *evsel, int cpu, int thread,
> > >        if (readn(*fd, count->values, size) <= 0)
> > >                return -errno;
> > >
> > > +     if (count->ena != count->run) {
> > > +             if (count->run != 0)
> > > +                     count->val = mul_u64_u64_div64(count->val, count->ena, count->run);
> > > +     }
> >
> > so I think perf stat expect raw values in there and does the
> > scaling by itself, please check following code:
> >
> > read_counters
> >   read_affinity_counters
> >     read_counter_cpu
> >       read_single_counter
> >         evsel__read_counter
> >
> >   perf_stat_process_counter
> >     process_counter_maps
> >       process_counter_values
> >         perf_counts_values__scale
> >
> >
> > perhaps we could export perf_counts_values__scale if it'd be any help
>
> Thank you for your comment.
>
> The purpose of this patch is to unify the counters obtained with
> perf_evsel__read() to scaled or unscaled values.
>
> perf_evsel__read() gets counter by perf_mmap__read_self() if RDPMC is
> available, else gets by readn(). In current implementation, caller
> gets scaled counter if goes through RDPMC path, otherwise gets unscaled
> counter via readn() path.
>
> However caller cannnot know which path were taken.
>
> If caller expects a raw value, I think the RDPMC path should also
> return an unscaled counter.
>
> diff --git a/tools/lib/perf/mmap.c b/tools/lib/perf/mmap.c
> index c89dfa5..aaa4579 100644
> --- a/tools/lib/perf/mmap.c
> +++ b/tools/lib/perf/mmap.c
> @@ -353,8 +353,6 @@ int perf_mmap__read_self(struct perf_mmap *map, struct perf_counts_values *count
>                 count->ena += delta;
>                 if (idx)
>                         count->run += delta;
> -
> -               cnt = mul_u64_u64_div64(cnt, count->ena, count->run);
>         }
>
>         count->val = cnt;
>
> Rob, do you have any comments?

Submit a proper patch with the above.

Rob