linux-kernel - Re: [RFC] perf: need to expose sched_clock to correlate user samples with kernel samples

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Message-ID: <51631510.6080401@linaro.org>
Date:	Mon, 08 Apr 2013 12:05:52 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Pawel Moll <pawel.moll@....com>
CC:	Richard Cochran <richardcochran@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Stephane Eranian <eranian@...gle.com>,
	David Ahern <dsahern@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Anton Blanchard <anton@...ba.org>,
	Will Deacon <Will.Deacon@....com>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	Pekka Enberg <penberg@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Robert Richter <robert.richter@....com>
Subject: Re: [RFC] perf: need to expose sched_clock to correlate user samples
 with kernel samples

On 04/08/2013 10:58 AM, Pawel Moll wrote:
> Now, before I spend time doing all this, a question to John, Peter,
> Stephane and the rest of the public - would a solution providing such
> userspace interface:
>
> 	fd = sys_perf_open()
> 	timestamp = clock_gettime((FD_TO_CLOCKID(fd), &ts)
>
> be acceptable to all?

So thinking this through further, I'm worried we may _not_ be able to 
eventually enable this to be a vdso as I had earlier hoped. Mostly 
because I'm not sure how the fd -> file -> clock lookup could be done in 
userland (any ideas?).

So this makes this approach mostly equivalent long term to the ioctl 
method, from a performance perspective. And makes the dynamic posix 
clockid somewhat less of a middle-ground compromise between the ioctl 
and generic constant clockid approach.

So while I'm not opposed to the sort of extention proposed above, I want 
to make sure introducing the new approach is worth the effort when 
compared with just adding an ioctl.

thanks
-john

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