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From:   "nakamura.shun@fujitsu.com" <nakamura.shun@fujitsu.com>
To:     Rob Herring <robh@kernel.org>
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
Thread-Topic: [PATCH 1/3] libperf: Add processing to scale the counters
 obtained during the read() system call when multiplexing
Thread-Index: AQHXmFsxBFGvQYUD8Uu1pPLYL2hYdauCbgJw
Date:   Tue, 24 Aug 2021 10:11:28 +0000
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References: <20210820093908.734503-1-nakamura.shun@fujitsu.com>
 <20210820093908.734503-2-nakamura.shun@fujitsu.com>
 <YSQBI+FZLBvKr088@robh.at.kernel.org>
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Hi, Rob=0A=
=0A=
> On Fri, Aug 20, 2021 at 06:39:06PM +0900, Shunsuke Nakamura wrote:=0A=
> > perf_evsel__read() scales counters obtained by RDPMC during multiplexin=
g, but=0A=
> > does not scale counters obtained by read() system call.=0A=
> > =0A=
> > Add processing to perf_evsel__read() to scale the counters obtained dur=
ing the=0A=
> > read() system call when multiplexing.=0A=
> =0A=
> Which one is right though? Changing what read() returns could break =0A=
> users, right? Or are you implying that the RDPMC path is correct and =0A=
> read() was not. More likely the former case since I wrote the latter.=0A=
=0A=
perf_evsel__read() returns both the count obtained by RDPMC and the count o=
btained=0A=
by the read() system call when multiplexed with RDPMC enabled.=0A=
=0A=
That is, there is a mix of scaled and unscaled values.=0A=
=0A=
As Rob says, when this patch is applied, rescaling the count obtained from=
=0A=
perf_evsel__read() during multiplexing will break the count.=0A=
=0A=
I think the easiest solution is to change the value you get from RDPMC to n=
ot scale =0A=
and let the user scale it, but I thought it would be a little inconvenient.=
=0A=
=0A=
Best Regards=0A=
Shunsuke=