linux-kernel - Re: [PATCH v3 0/3] perf: User/kernel time correlation and event generation

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Message-ID: <CALCETrXGoevmD_avz5sQfbbD624vpLW5=-8ovzTPT_5wzNFnVA@mail.gmail.com>
Date:	Mon, 3 Nov 2014 16:58:32 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Pawel Moll <pawel.moll@....com>
Cc:	Richard Cochran <richardcochran@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	John Stultz <john.stultz@...aro.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Christopher Covington <cov@...eaurora.org>,
	Namhyung Kim <namhyung@...nel.org>,
	David Ahern <dsahern@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tomeu Vizoso <tomeu@...euvizoso.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Pawel Moll <mail@...elmoll.com>
Subject: Re: [PATCH v3 0/3] perf: User/kernel time correlation and event generation

On Mon, Nov 3, 2014 at 4:28 PM, Pawel Moll <pawel.moll@....com> wrote:
> From: Pawel Moll <mail@...elmoll.com>
> Thomas suggested solution which gets down to my original proposal for
> sched/monotonic clock correlation - an additional sample type so events
> can be "double stamped" using different clock sources providing
> synchronisation points for later time approximation. I've just extended
> the implementation with configuration value to select the clock source.
> If the first patch (making perf timestamps monotonic) gets accepted,
> there will be no immediate need for this one, but I'd like to gain some
> feedback anyway.
>

I have nothing intelligent to add to the potentional Thomas/Ingo
showdown, but I do have a related thought. :)

If you're going to add double-stamped packets, can you also add a
syscall to read multiple clocks at once, atomically?  Or can you
otherwise add a non-perf mechanism to get at this data?

Because the realtime to monotonic offset is really quite useful for
things like this, and it seems silly to make people actually open a
perf_event to get at it.

--Andy
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