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

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Date:	Mon, 3 Nov 2014 17:25:38 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	John Stultz <john.stultz@...aro.org>
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 5:11 PM, John Stultz <john.stultz@...aro.org> wrote:
> 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.

My only real thought here is that, if perf is going to try to do this,
then presumably it should be reasonably integrated w/ the core timing
code.  I.e. if perf does this, then presumably the core code should
know about it and there should be a core interface to it.

--Andy

>
> thanks
> -john



-- 
Andy Lutomirski
AMA Capital Management, LLC
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