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

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Date:	Wed, 03 Apr 2013 18:35:02 +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 Wed, 2013-04-03 at 18:29 +0100, John Stultz wrote:
> On 04/03/2013 10:19 AM, Pawel Moll wrote:
> > On Tue, 2013-04-02 at 17:19 +0100, John Stultz wrote:
> >> 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 ;).
> > Ok, so how about the code below?
> >
> > There are two distinct parts of the "solution":
> >
> > 1. The dynamic posix clock, as you suggested. Then one can get the perf
> > timestamp by doing:
> >
> > 	clock_fd = open("/dev/perf-clock", O_RDONLY);
> > 	clock_gettime(FD_TO_CLOCKID(clock_fd), &ts)
> >
> > 2. A sort-of-hack in the get_posix_clock() function making it possible
> > to do the same using the perf event file descriptor, eg.:
> >
> > 	fd = sys_perf_event_open(&attr, -1, 0, -1, 0);
> > 	clock_gettime(FD_TO_CLOCKID(fd), &ts)
> 
> #2 makes my nose wrinkle. 

To make myself clear: I consider the code as it is a hack.

> Forgive me for being somewhat ignorant on the 
> perf interfaces, but why is the second portion necessary or beneficial?

My thinking: the perf syscall returns a file descriptor already, so it
would make sense to re-use it in the clock_gettime() call instead of
jumping through loops to open a character device file, which may not
exist at all (eg. no udev) or may be placed or named in a random way
(eg. some local udev rule).

I'm open for different opinions :-)

Pawel


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