linux-kernel - Re: [PATCH 0/2] perf/x86: Add ability to sample TSC

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Message-ID: <54E61FFF.2010201@intel.com>
Date:	Thu, 19 Feb 2015 19:40:15 +0200
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	John Stultz <john.stultz@...aro.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>,
	Namhyung Kim <namhyung@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Pawel Moll <pawel.moll@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 0/2] perf/x86: Add ability to sample TSC

On 19/02/2015 7:24 p.m., John Stultz wrote:
> On Thu, Feb 19, 2015 at 4:11 AM, Adrian Hunter <adrian.hunter@...el.com> wrote:
>> Hi
>>
>> With the advent of switching perf_clock to CLOCK_MONOTONIC,
>> it will not be possible to convert perf_clock directly to/from
>> TSC. So add the ability to sample TSC instead.
>>
>>
>> Adrian Hunter (2):
>>        perf: Sample additional clock value
>>        perf/x86: Provide TSC for PERF_SAMPLE_CLOCK_ARCH
>
> This doesn't seem very portable. The CLOCK_MONOTONIC_RAW clockid was
> added to provide a arch-neutral abstraction of a free-running hardware
> counter that isn't affected by adjtimex slewing (though like any
> counter, it will be affected by non-constant drift).
>
> You might consider looking at that if the short term slew adjustments
> (which result in more accurate timings in the long term) are
> problematic for you.

This is for Intel Processor Trace - which Peter has already
rightly chastised me for not making plain.

Intel Processor Trace (Intel PT) is a trace that is hardware generated. The
hardware does not know about linux or its clocks, so the timestamps
are TSC. I think ARM might have the same issue with ETM or such. i.e. the
need to synchronize a hardware timestamp with a perf event.

There is a description of Intel PT in the Intel Architecture manuals.

http://www.intel.com/content/www/us/en/processors/architectures-software-developer-manuals.html

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