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

From: John Stultz <john.stultz@linaro.org>
To: Pawel Moll <pawel.moll@arm.com>
Cc: 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>,
	Richard Cochran <richardcochran@gmail.com>
Subject: Re: [RFC] perf: need to expose sched_clock to correlate user samples with kernel samples
Date: Wed, 03 Apr 2013 10:50:57 -0700	[thread overview]
Message-ID: <515C6C01.9070905@linaro.org> (raw)
In-Reply-To: <1365010502.26858.32.camel@hornet>

On 04/03/2013 10:35 AM, Pawel Moll wrote:
> 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 :-)

Cc'ing Richard for his thoughts here.

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'm guessing the private_data on the perf file is already used?

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.

thanks
-john