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Message-ID: <51631510.6080401@linaro.org>
Date: Mon, 08 Apr 2013 12:05:52 -0700
From: John Stultz <john.stultz@linaro.org>
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To: Pawel Moll <pawel.moll@arm.com>
CC: Richard Cochran <richardcochran@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Stephane Eranian <eranian@google.com>, David Ahern <dsahern@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, "mingo@elte.hu" <mingo@elte.hu>,
        Paul Mackerras <paulus@samba.org>, Anton Blanchard <anton@samba.org>,
        Will Deacon <Will.Deacon@arm.com>,
        "ak@linux.intel.com" <ak@linux.intel.com>,
        Pekka Enberg <penberg@gmail.com>, Steven Rostedt <rostedt@goodmis.org>,
        Robert Richter <robert.richter@amd.com>
Subject: Re: [RFC] perf: need to expose sched_clock to correlate user samples
 with kernel samples
References: <51586315.7080006@gmail.com> <5159D221.70304@linaro.org>  <1364889256.16858.1.camel@laptop> <515B0502.8070408@linaro.org>  <1365009558.26858.19.camel@hornet> <515C66FE.7030501@linaro.org>  <1365010502.26858.32.camel@hornet> <515C6C01.9070905@linaro.org>  <1365092963.26858.107.camel@hornet> <1365185813.25942.12.camel@hornet>  <20130406110507.GC7572@netboy> <1365443897.24306.12.camel@hornet>
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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