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 08:14:20 -0600
From:	David Ahern <dsahern@...il.com>
To:	Stephane Eranian <eranian@...gle.com>
CC:	John Stultz <john.stultz@...aro.org>,
	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>,
	Robert Richter <robert.richter@....com>
Subject: Re: [RFC] perf: need to expose sched_clock to correlate user samples
 with kernel samples

On 4/3/13 8:00 AM, Stephane Eranian wrote:
>> What's the advantage of changing apps -- like the JIT compiler -- to emit
>> perf based timestamps versus having perf emit existing timestamps? ie.,
>> monotonic and realtime clocks already have vdso mappings for userspace with
>> well known performance characteristics. Why not have perf convert its
>> perf_clock timestamps into monotonic or realtime when dumping events?
>>
> Can monotonic timestamps be obtained from NMI context in the kernel?

I don't understand the context of the question.

I am not suggesting perf_clock be changed. I am working on correlating 
existing perf_clock timestamps to clocks typically used by apps 
(REALTIME and time-of-day but also applies to MONOTONIC).

You are wanting the reverse -- have apps emit perf_clock timestamps. I 
was just wondering what is the advantage of this approach?

David

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