linux-kernel - Re: [PATCH v2 1/2] perf: Add sampling of the raw monotonic clock

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Date:	Mon, 29 Sep 2014 15:47:36 +0100
From:	Pawel Moll <pawel.moll@....com>
To:	David Ahern <dsahern@...il.com>
Cc:	Namhyung Kim <namhyung@...nel.org>,
	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>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] perf: Add sampling of the raw monotonic clock

On Fri, 2014-09-26 at 20:25 +0100, David Ahern wrote:
> On 9/26/14, 9:05 AM, Pawel Moll wrote:
> > To do the correlation you need both timestamps to be "taken"
> > simultaneously:
> >
> >          perf event     user event
> >         -----O--------------+-------------O------> t_mono
> >              :              |             :
> >              :              V             :
> >         -----O----------------------------O------> t_perf
> >
> > Of course it's not possible get both values literally at the same time,
> > but placing them in a atomic context a couple of instructions from each
> > other still gives pretty good results. The larger this distance is, the
> 
> An early patchset on this topic added the realtime clock as an event and 
> an ioctl was used to push a sample into the event stream. 

Yeah, I remember. If I remember correctly correctly the pushback was on
a custom event type, right?

Generally speaking I don't mind any solution that we'll get us to the
place both you and I want to be (just being able to time stamp some
performance data in userspace, how difficult can this be! ;-) but I like
the flexibility of an extra sample - one can pick and mix events and
samples at one's leisure.

> In that case 
> you have wall clock and perf-clock samples taken in the same kernel 
> context and about as close together as you can get.

Yep, that's what I was saying - we can't quite get two timestamps at the
*same*, but getting them within a single atomic block of instructions
gives reasonable accuracy.

Thanks!

Pawel

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