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

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Date:	Tue, 02 Apr 2013 17:34:49 +0100
From:	Pawel Moll <pawel.moll@....com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	David Ahern <dsahern@...il.com>,
	Stephane Eranian <eranian@...gle.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 Tue, 2013-04-02 at 17:19 +0100, John Stultz wrote:
> I still think exposing the perf clock to userland is a bad idea, and 
> would much rather the kernel provide timestamp data in the logs 
> themselves to make the logs useful. But if we're going to have to do 
> this via a clockid, I'm going to want it to be done via a dynamic posix 
> clockid, so its clear its tightly tied with perf and not considered a 
> generic interface (and I can clearly point folks having problems to the 
> perf maintainers ;).

Hm. 15 mins ago I didn't know about dynamic posix clocks existence at
all ;-)

I feel that the idea of opening a magic character device to obtain a
magic number to be used with clock_gettime() to get the timestamp may
not be popular, but maybe (just a thought) we could somehow use the file
descriptor obtained by the sys_perf_open() itself? How different would
it be from the ioctl(*_GET_TIME) I'm not sure, but I'll try to research
the idea. Counts as number 4 (5?) on my list ;-)

Paweł


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