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From: Rob Herring <robh@kernel.org>
To: "nakamura.shun@fujitsu.com" <nakamura.shun@fujitsu.com>
Cc: "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, 31 Aug 2021 07:26:18 -0500	[thread overview]
Message-ID: <CAL_JsqLKD0DZT63MQ9vTumOf1bfRrU9Bt9Qa_GTb1siRsK+Z+g@mail.gmail.com> (raw)
In-Reply-To: <OSBPR01MB4600288E176A169C07BF543EF7C59@OSBPR01MB4600.jpnprd01.prod.outlook.com>

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?

Rob

  parent reply	other threads:[~2021-08-31 12:26 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 [this message]
2021-09-07 23:59         ` Ian Rogers
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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