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

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Date:	Thu, 04 Apr 2013 15:26:39 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Stephane Eranian <eranian@...gle.com>
CC:	David Ahern <dsahern@...il.com>, Pawel Moll <pawel.moll@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	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>
Subject: Re: [RFC] perf: need to expose sched_clock to correlate user samples
 with kernel samples

On 04/04/2013 01:12 AM, Stephane Eranian wrote:
> On Wed, Apr 3, 2013 at 7:57 PM, John Stultz <john.stultz@...aro.org> wrote:
>> I'm not sure I follow this. If perf exported data came with CLOCK_MONOTONIC
>> timestamps, no correlation would need to be exposed.  perf would just have
>> to do the extra overhead of doing the conversion on export.
> There is no explicit export operation in perf.  You record a sample when
> the counter overflows and generates an NMI interrupt. In the NMI interrupt
> handler, the sample record is written to the sampling buffer. That is when
> the timestamp is generated. The sampling buffer is directly accessible to
> users via mmap(). The perf tool just dumps the raw sampling buffer into
> a file, no sample record is modified or even looked at. The processing
> of the samples is done offline (via perf report) and could be done on
> another machine. In other words, the perf.data file is self-contained.
Ah. Ok, I didn't realize perfs buffers were directly mmaped.  I was 
thinking perf could do the translation not at NMI time but when the 
buffer was later read by the application.  That helps explain some of 
the constraints.

thanks
-john
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