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

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Date:	Wed, 20 Feb 2013 11:29:20 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Stephane Eranian <eranian@...gle.com>,
	Pawel Moll <pawel.moll@....com>,
	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 Tue, 2013-02-19 at 10:25 -0800, John Stultz wrote:
> So describe how the perf time domain is different then
> CLOCK_MONOTONIC_RAW.

The primary difference is that the trace/sched/perf time domain is not
strictly monotonic, it is only locally monotonic -- that is two time
stamps taken on the same cpu are guaranteed to be monotonic.

Furthermore, to make it useful, there's an actual bound on the inter-cpu
drift (implemented by limiting the drift to CLOCK_MONOTONIC).

Additionally -- to increase use -- we also added a monotonic sync point
when cpu A queries time of cpu B.

> My concern here is that we're basically creating a kernel interface
> that 
> exports implementation-defined semantics (again: whatever perf does 
> right now). And I think folks want to do this, because adding
> CLOCK_PERF 
> is easier then trying to:
> 
> 1) Get a lock-free method for accessing CLOCK_MONOTONIC_RAW
> 
> 2) Having perf interpolate its timestamps to CLOCK_MONOTONIC, or 
> CLOCKMONOTONIC_RAW when it exports the data

Mostly cheaper, not easier. Given unstable TSC, MONOTONIC will have to
fall back to another clock source (hpet, acpi_pm and other assorted
crap).

In order to avoid this, we'd had to relax the requirements. Using
anything other than TSC is simply not an option.

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