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Date: Wed, 11 Dec 2013 12:25:26 +0100
From: Ingo Molnar <mingo@kernel.org>
To: David Ahern <dsahern@gmail.com>
Cc: linux-kernel@vger.kernel.org, Pawel Moll <pawel.moll@arm.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
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        Namhyung Kim <namhyung@kernel.org>,
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        John Stultz <john.stultz@linaro.org>,
        Sonny Rao <sonnyrao@chromium.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] perf: POSIX CLOCK_PERF to report current time value
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* David Ahern <dsahern@gmail.com> wrote:

> From: Pawel Moll <pawel.moll@arm.com>
> 
> To co-relate user space events with the perf events stream
> a current (as in: "what time(stamp) is it now?") time value
> must be made available.

So I'm wondering about your earlier approach posted here:

  https://lkml.org/lkml/2011/6/7/636

I'd modify that patch the following way: instead of tracking each 
separate reason, perhaps only track timekeeping_update().

Such a tracepoint, combined with PERF_SAMPLE_TIME, would very 
accurately track external changes to GTOD (xtime).

That leaves us with tracking/correlating the regular flow of time, 
which could be achieved by another tracepoint in:

   kernel/time/timekeeping.c::do_timer()

So only two new tracepoints are needed AFAICS - and the tracepoints 
would obviously be useful for other (debugging) purposes as well. 
Would that solve the wall-clock correlation problem adequately?

Thanks,

	Ingo