linux-kernel - Re: [PATCH v4 3/3] perf: Sample additional clock value

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Message-ID: <54C28D71.6040603@gmail.com>
Date:	Fri, 23 Jan 2015 11:05:37 -0700
From:	David Ahern <dsahern@...il.com>
To:	Pawel Moll <pawel.moll@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	John Stultz <john.stultz@...aro.org>
CC:	Richard Cochran <richardcochran@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	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>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tomeu Vizoso <tomeu@...euvizoso.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [PATCH v4 3/3] perf: Sample additional clock value

On 1/23/15 10:06 AM, Pawel Moll wrote:
> As far as I understand (John?) POSIX timers can be used on any clockid?
> So it would be possible to obtain a dynamic clock id, for example for my
> exotic trace hardware (by any means necessary, like opening a char
> device) and create a timer firing every 1 ms (in the trace time domain).
> Than this event would be somehow associated with a perf session (for
> example, by passing the timerid via perf's ioctl) and then, every when
> timer fires, a perf record (something like PERF_RECORD_TIMER?)
> containing the timer/clock's value*and*  the normal perf timestamp,
> would be injected into the circular buffer.

Like this: https://lkml.org/lkml/2011/2/27/158 ? note the date -- 4 
years ago. This is has been dragging on for a long time.

A few problems with that approach:
1. I would like to see a sample generated immediately to get the 
perf_clock -> timeofday correlation immediately rather than have to wait 
N (milli)seconds and have perf scan forward through an M-(giga)byte file 
looking for the one sample that gives the correlation.

I tried to address that problem with an ioctl to force a sample into the 
stream:
    https://lkml.org/lkml/2011/2/27/159
it did not go over very well.

2. there is a risk that the realtime samples dominate a stream.

Another issue that has been raised is updates to xtime by ntp / user. I 
have suggested tracepoints to catch those:
     https://lkml.org/lkml/2011/6/7/636
I don't believe there were ever any comments on the tracepoints.

David
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