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

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Date:	Wed, 06 Feb 2013 18:17:42 +0000
From:	Pawel Moll <pawel.moll@....com>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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>,
	tglx <tglx@...utronix.de>, John Stultz <john.stultz@...aro.org>
Subject: Re: [RFC] perf: need to expose sched_clock to correlate user
 samples with kernel samples

On Tue, 2013-02-05 at 22:13 +0000, Stephane Eranian wrote:
> The app requesting the timestamp may not necessarily have an active
> perf session. And by that I mean, it may not be self-monitoring. But it
> could be monitored by an external tool such as perf, without necessary
> knowing it.

Fair enough - I guess eg. JIT engine could generate a timestamped
information about generated symbols which could be then merged with a
stream recorded by the perf tool by yet another tool.

> The timestamp is global or at least per-cpu. It is not tied to a particular
> active event.

I know that the implementation does it now, but is it actually specified
as such? I could imagine a situation where perf_clock() returns time
elapsed since the "current" sys_perf_event_open()... (not that I'd like
it ;-)

Paweł


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