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

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Date:	Mon, 3 Nov 2014 17:11:53 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Pawel Moll <pawel.moll@....com>,
	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>,
	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 4:58 PM, Andy Lutomirski <luto@...capital.net> wrote:
> 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.

So this comes up periodically, but I don't think I've seen a interface
proposal that was decent yet.

Also, if you want to read multiple clocks at once, do you stop at two,
or three, or... there's possibly quite a few.  Additionally some
clocks may not be possible to read atomically (perf/sched clock and
system time for example may be based on different underlying
clocksources). The general idea feels like its creeping towards some
"atomically expose all timekeeping state" mega-interface.

I've got some thoughts on what a possible interface that wouldn't be
awful could look like, but I'm still hesitant because I don't really
know if exposing this sort of data is actually a good idea long term.

thanks
-john
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