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

From: Pawel Moll <pawel.moll@arm.com>
To: Richard Cochran <richardcochran@gmail.com>
Cc: John Stultz <john.stultz@linaro.org>,
	Peter Zijlstra <peterz@infradead.org>,
	David Ahern <dsahern@gmail.com>,
	Stephane Eranian <eranian@google.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
Date: Thu, 04 Apr 2013 17:33:22 +0100	[thread overview]
Message-ID: <1365093202.26858.111.camel@hornet> (raw)
In-Reply-To: <20130404073700.GA1860@localhost.localdomain>

On Thu, 2013-04-04 at 08:37 +0100, Richard Cochran wrote:
> > I get the reasoning around reusing the fd we already have, but is
> > the possibility of a dynamic chardev pathname really a big concern?
> 
> I have been following this thread, and, not knowing very much about
> perf, I would think that the userland can easily open a second file
> (the dynamic posix clock chardev) in order to get these time stamps.

Sure it can - I've tested it. It's just a bit cumbersome in my opinion
(there is nothing else perf-related in /dev). I can agree to disagree if
you think otherwise :-)

> > Maybe can we extend the dynamic posix clock code to work on more
> > then just the chardev? Although I worry about multiplexing too much
> > functionality on the file.
> 
> I don't yet see a need for that, but if we do, then it should work in
> a generic way, and not as a list of special cases, like we saw in the
> patch.

By all means - and even more generic way than it is now (why character
devices not any other file?). I'll give it a try.

Paweł