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

From: Pawel Moll <pawel.moll@arm.com>
To: John Stultz <john.stultz@linaro.org>
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>
Subject: Re: [RFC] perf: need to expose sched_clock to correlate user samples with kernel samples
Date: Tue, 02 Apr 2013 17:34:49 +0100	[thread overview]
Message-ID: <1364920489.3019.26.camel@hornet> (raw)
In-Reply-To: <515B0502.8070408@linaro.org>

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