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, 24 Aug 2021 10:11:28 +0000
From:   "nakamura.shun@...itsu.com" <nakamura.shun@...itsu.com>
To:     Rob Herring <robh@...nel.org>
CC:     "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

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.

Best Regards
Shunsuke