linux-kernel - Re: [PATCH v2] perf metrics: Add literal for system TSC frequency

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Message-ID: <76dce5d3-6c69-e403-c4d9-f882967a4467@linux.intel.com>
Date:   Fri, 27 May 2022 07:54:19 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ian Rogers <irogers@...gle.com>
Cc:     perry.taylor@...el.com, caleb.biggers@...el.com,
        kshipra.bopardikar@...el.com,
        Kan Liang <kan.liang@...ux.intel.com>,
        Zhengjun Xing <zhengjun.xing@...ux.intel.com>,
        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>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] perf metrics: Add literal for system TSC frequency


On 5/27/2022 2:49 AM, Peter Zijlstra wrote:
> On Thu, May 26, 2022 at 09:04:07PM -0700, 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 accurate at least on Intel
>> processors.

It would be better to use CPUID if available which is far more accurate. 
Also I believe newer Intel CPUs drop the frequency in the brand string.

BTW it also has the problem that if perf script is run on some other 
system to compute metrics it won't work.

>
> This all seems bonghits inspired... and perf actually does expose the
> tsc frequency. What do you think is in perf_event_mmap_page::time_* ?


That's not really available to perf stat, which is the primary metrics user.


-Andi