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

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Date:	Wed, 3 Apr 2013 16:22:47 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	David Ahern <dsahern@...il.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>
Subject: Re: [RFC] perf: need to expose sched_clock to correlate user samples
 with kernel samples

On Wed, Apr 3, 2013 at 4:14 PM, David Ahern <dsahern@...il.com> wrote:
> 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).
>
But for that, you'd need to expose to users the correlation between
the two clocks.
And now you'd fixed two clock sources definitions not just one.

> You are wanting the reverse -- have apps emit perf_clock timestamps. I was
> just wondering what is the advantage of this approach?
>
Well, that's how I interpreted your question ;-<

If you could have perf_clock use monotonic then we would not have this
discussion.
The correlation would be trivial.
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