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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>,
	Arnaldo Carvalho de Melo <acme@redhat.com>,
	Mike Galbraith <efault@gmx.de>, Jiri Olsa <jolsa@redhat.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Stephane Eranian <eranian@google.com>,
	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
Date: Wed, 11 Dec 2013 12:25:26 +0100	[thread overview]
Message-ID: <20131211112526.GC25541@gmail.com> (raw)
In-Reply-To: <1386707259-21725-1-git-send-email-dsahern@gmail.com>


* 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

  parent reply	other threads:[~2013-12-11 11:25 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-12-10 20:27 [PATCH] perf: POSIX CLOCK_PERF to report current time value David Ahern
2013-12-10 20:44 ` John Stultz
2013-12-11 12:07   ` Ingo Molnar
2013-12-11 19:37     ` John Stultz
2013-12-11 11:25 ` Ingo Molnar [this message]
2013-12-11 11:40   ` Ingo Molnar

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