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

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Date:	Thu, 19 Feb 2015 09:50:21 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Adrian Hunter <adrian.hunter@...el.com>
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>,
	Mathieu Poirier <mathieu.poirier@...aro.org>
Subject: Re: [PATCH 0/2] perf/x86: Add ability to sample TSC

On Thu, Feb 19, 2015 at 9:40 AM, Adrian Hunter <adrian.hunter@...el.com> wrote:
> 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

Cc'ing Mathieu since he might be familiar w/ any similar issues w/ Coresight.

thanks
-john
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