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From: Ian Rogers <irogers@google.com>
To: Rob Herring <robh@kernel.org>
Cc: "nakamura.shun@fujitsu.com" <nakamura.shun@fujitsu.com>,
	"peterz@infradead.org" <peterz@infradead.org>,
	"mingo@redhat.com" <mingo@redhat.com>,
	"acme@kernel.org" <acme@kernel.org>,
	"mark.rutland@arm.com" <mark.rutland@arm.com>,
	"alexander.shishkin@linux.intel.com" 
	<alexander.shishkin@linux.intel.com>,
	"jolsa@redhat.com" <jolsa@redhat.com>,
	"namhyung@kernel.org" <namhyung@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-perf-users@vger.kernel.org"
	<linux-perf-users@vger.kernel.org>
Subject: Re: [PATCH 1/3] libperf: Add processing to scale the counters obtained during the read() system call when multiplexing
Date: Tue, 7 Sep 2021 16:59:42 -0700	[thread overview]
Message-ID: <CAP-5=fW_g9JHKWQoNiwNeSN8MjJ1OA7qLb3JD3ErEi1be4DEiQ@mail.gmail.com> (raw)
In-Reply-To: <CAL_JsqLKD0DZT63MQ9vTumOf1bfRrU9Bt9Qa_GTb1siRsK+Z+g@mail.gmail.com>

On Tue, Aug 31, 2021 at 5:26 AM Rob Herring <robh@kernel.org> wrote:
>
> On Tue, Aug 24, 2021 at 5:12 AM nakamura.shun@fujitsu.com
> <nakamura.shun@fujitsu.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

  reply	other threads:[~2021-09-07 23:59 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-20  9:39 [PATCH 0/3] libperf: Add support for scaling counters obtained from the read() system call during multiplexing Shunsuke Nakamura
2021-08-20  9:39 ` [PATCH 1/3] libperf: Add processing to scale the counters obtained during the read() system call when multiplexing Shunsuke Nakamura
2021-08-23 20:12   ` Rob Herring
2021-08-24 10:11     ` nakamura.shun
2021-08-31  8:58       ` nakamura.shun
2021-08-31 12:26       ` Rob Herring
2021-09-07 23:59         ` Ian Rogers [this message]
2021-09-17  8:04           ` nakamura.shun
2021-08-20  9:39 ` [PATCH 2/3] libperf tests: Fix verbose printing Shunsuke Nakamura
2021-08-23 20:26   ` Rob Herring
2021-08-24 18:06     ` Arnaldo Carvalho de Melo
2021-08-20  9:39 ` [PATCH 3/3] libperf tests: Add test_stat_multiplexing test Shunsuke Nakamura

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