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

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Date:	Tue, 13 Nov 2012 15:58:57 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Stephane Eranian <eranian@...gle.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>,
	Robert Richter <robert.richter@....com>,
	tglx <tglx@...utronix.de>
Subject: Re: [RFC] perf: need to expose sched_clock to correlate user
 samples with kernel samples

On Fri, 2012-11-09 at 18:04 -0800, John Stultz wrote:
> On 10/16/2012 10:23 AM, Peter Zijlstra wrote:

> > I've no problem with adding CLOCK_PERF (or another/better name).
> Hrm. I'm not excited about exporting that sort of internal kernel 
> details to userland.
> 
> The behavior and expectations from sched_clock() has changed over the 
> years, so I'm not sure its wise to export it, since we'd have to 
> preserve its behavior from then on.
> 
> Also I worry that it will be abused in the same way that direct TSC 
> access is, where the seemingly better performance from the more 
> careful/correct CLOCK_MONOTONIC would cause developers to write fragile 
> userland code that will break when moved from one machine to the next.
> 
> I'd probably rather perf output timestamps to userland using sane clocks 
> (CLOCK_MONOTONIC), rather then trying to introduce a new time domain to 
> userland.   But I probably could be convinced I'm wrong.

I'm surprised that perf has its own clock anyway. But I would like to
export the tracing clocks. We have three (well four) of them:

trace_clock_local() which is defined to be a very fast clock but may not
be synced with other cpus (basically, it just calls sched_clock).

trace_clock() which is not totally serialized, but also not totally off
(between local and global). This uses local_clock() which is the same
thing that perf_clock() uses.

trace_clock_global() which is a monotonic clock across CPUs. It's much
slower than the above, but works well when you require synced
timestamps.

There's also trace_clock_counter() which isn't even a clock :-)  It's
just a incremental atomic counter that goes up every time it's called.
This is the most synced clock, but is absolutely meaningless for
timestamps. It's just a way to show ordered events.

-- Steve

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