linux-kernel - Re: [PATCH] perf metrics: Add literal for system TSC frequency

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Message-ID: <CAP-5=fVxFy36wZKu+M199qic1VYWo_Z9GQ+jBAp6ZkBHTHv4Ww@mail.gmail.com>
Date:   Wed, 2 Feb 2022 23:58:02 -0800
From:   Ian Rogers <irogers@...gle.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     vineet.singh@...el.com, perry.taylor@...el.com,
        caleb.biggers@...el.com, asaf.yaffe@...el.com,
        kshipra.bopardikar@...el.com,
        Kan Liang <kan.liang@...ux.intel.com>,
        Zhengjun Xing <zhengjun.xing@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        James Clark <james.clark@....com>,
        John Garry <john.garry@...wei.com>,
        linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH] perf metrics: Add literal for system TSC frequency

On Wed, Feb 2, 2022 at 7:59 PM Andi Kleen <ak@...ux.intel.com> wrote:
>
>
> On 2/2/2022 5:07 PM, Ian Rogers wrote:
> > Such a literal is useful to calculate things like the average frequency
> > [1]. The TSC frequency isn't exposed by sysfs although some experimental
> > drivers look to add it [2]. This change computes the value using the
> > frequency in /proc/cpuinfo which is accruate at least on Intel
> > processors.
>
> FWIW the information in /proc/cpuinfo is going away.
>
> You should really use CPUID for this, which has a new leaf for this.
> Unfortunately there
>
> were a few systems were it was inaccurate, but I suppose you could still
> prefer /proc/cpuinfo
>
> Also your method will not work when the metrics are used through perf
> stat report or perf script
>
> and run on another system. For the perf script case there is support for
> putting the TSC into
>
> perf.data (that is needed for PT), but the header is not dumped by
> default unfortunately.
>
> So likely you would need some new mechanism for that.

Thanks Andi,

yep metrics with perf stat record are broken on a number of fronts and
this will be another that will need a fix. It looks like the cpuid
approach won't work pre-Skylake so sysfs could act as a fallback, but
I don't have an ETA on when that will be available. We could have
additional warnings when reading /proc/cpuinfo fails. Wdyt?

Thanks,
Ian

> -Andi
>