linux-kernel - Re: [PATCH] perf: POSIX CLOCK_PERF to report current time value

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Date:	Wed, 11 Dec 2013 12:25:26 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	David Ahern <dsahern@...il.com>
Cc:	linux-kernel@...r.kernel.org, Pawel Moll <pawel.moll@....com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Mike Galbraith <efault@....de>, Jiri Olsa <jolsa@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Stephane Eranian <eranian@...gle.com>,
	John Stultz <john.stultz@...aro.org>,
	Sonny Rao <sonnyrao@...omium.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] perf: POSIX CLOCK_PERF to report current time value

* David Ahern <dsahern@...il.com> wrote:

> From: Pawel Moll <pawel.moll@....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
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