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

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Date:	Thu, 19 Feb 2015 09:24:17 -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>
Subject: Re: [PATCH 0/2] perf/x86: Add ability to sample TSC

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.

thanks
-john
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