linux-kernel - Re: [PATCH 1/3] libperf: Add processing to scale the counters obtained during the read() system call when multiplexing

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Date:   Tue, 7 Sep 2021 16:59:42 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Rob Herring <robh@...nel.org>
Cc:     "nakamura.shun@...itsu.com" <nakamura.shun@...itsu.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>,
        "jolsa@...hat.com" <jolsa@...hat.com>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>
Subject: Re: [PATCH 1/3] libperf: Add processing to scale the counters
 obtained during the read() system call when multiplexing

On Tue, Aug 31, 2021 at 5:26 AM Rob Herring <robh@...nel.org> wrote:
>
> On Tue, Aug 24, 2021 at 5:12 AM nakamura.shun@...itsu.com
> <nakamura.shun@...itsu.com> wrote:
> >
> > Hi, Rob
> >
> > > On Fri, Aug 20, 2021 at 06:39:06PM +0900, Shunsuke Nakamura wrote:
> > > > 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.
> > >
> > > Which one is right though? Changing what read() returns could break
> > > users, right? Or are you implying that the RDPMC path is correct and
> > > read() was not. More likely the former case since I wrote the latter.
> >
> > perf_evsel__read() returns both the count obtained by RDPMC and the count obtained
> > by the read() system call when multiplexed with RDPMC enabled.
> >
> > That is, there is a mix of scaled and unscaled values.
> >
> > As Rob says, when this patch is applied, rescaling the count obtained from
> > perf_evsel__read() during multiplexing will break the count.
> >
> > I think the easiest solution is to change the value you get from RDPMC to not scale
> > and let the user scale it, but I thought it would be a little inconvenient.
>
> Agreed, unless someone else has an opinion. It would be good to do the
> scaling in libperf with the optimized math op, but I assume there's
> some reason the user may need unscaled values?

Hi, something I've mentioned on other threads [1] is that running may
be zero due to multiplexing but enabled be greater. This can lead to a
divide by zero when scaling. Giving the ratio to the caller gives more
information - I may be misunderstanding this thread, apologies if so.

Thanks,
Ian

[1] https://lore.kernel.org/lkml/CAL_JsqKc_qFA59L9e-xXOhE4yBTdVg-Ea9EddimWwqj3XXxhGw@mail.gmail.com/

> Rob