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

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Date:	Tue, 04 Nov 2014 09:01:31 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	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 Monday 03 November 2014 17:11:53 John Stultz wrote:
> 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.
 
I was also thinking (while working on an unrelated patch) we could use
a system call like

int clock_getoffset(clockid_t clkid, struct timespec *offs);

that returns the current offset between CLOCK_REALTIME and the
requested timebase. It is of course racy, but so is every use
of CLOCK_REALTIME. We could also use a reference other than
CLOCK_REALTIME that might be more stable, but passing two arbitrary
clocks as input would make this much more complex to implement.

	Arnd
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