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Date:	Mon, 26 Oct 2009 12:05:09 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Pierre-Marc Fournier <pierre-marc.fournier@...ymtl.ca>,
	Ingo Molnar <mingo@...e.hu>, GeunSik Lim <leemgs1@...il.com>,
	Zhaolei <zhaolei@...fujitsu.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Jesper Juhl <jj@...osbits.net>, Adrian Bunk <bunk@...sta.de>,
	Harvey Harrison <harvey.harrison@...il.com>,
	"Robert P. J. Day" <rpjday@...dspring.com>,
	Jaswinder Singh Rajput <jaswinderrajput@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-kernel@...r.kernel.org,
	Dominique Toupin <dominique.toupin@...csson.com>,
	Michel Dagenais <michel.dagenais@...ymtl.ca>
Subject: Re: Relicensing tracepoints and markers to Dual LGPL v2.1/GPL
	v2,headers to Dual BSD/GPL

* Arjan van de Ven (arjan@...radead.org) wrote:
> On Mon, 26 Oct 2009 09:17:49 -0400
> Pierre-Marc Fournier <pierre-marc.fournier@...ymtl.ca> wrote:
> 
> > Ingo Molnar wrote:
> > > 
> > > But i also disagree with it on a technical level: code duplication
> > > is _bad_. Why does the code have to be duplicated in user-space
> > > like that? I'd like Linux tracing code to be in the kernel repo.
> > > Why isnt this done properly, as part of the kernel project - to
> > > make sure it all stays in sync?
> > > 
> > 
> > If you mean that this code should solely be used inside the kernel,
> > then what you propose technically does not work. There is a very high
> > cost to accessing kernel code from userspace. 
> 
> yeah 100 cycles is insanely high, that's at least the equivalent of...
> say one cache miss.
> 

Hi Arjan,

Maybe it's just me, but if you are talking of system calls performed
with sysenter, my own machine (with an Intel Xeon E5405 CPU) seems to
disagree with your estimations:

For a system call doing basically _nothing_:

Cache-cold system call:
Time for a system call (returns after a simple test) :
TSC diff : 6426 cycles
time diff : 3212.53 ns

Cache-hot system call:
Time for system call (10000 calls) (returns after a simple test) :
TSC diff : 599.893 cycles per call
time diff : 299.903 ns per call

Otherwise, if you are instead referring to a vDSO, then the contexts is
slightly different, and it becomes cheaper, but it does not allow to
share code as much between the in-kernel and vDSO implementations,
mainly due to different locking required. Moreover, I wonder about the
maximum vDSO size that can be ported across multiple architectures. It
has only been used for really tiny pieces of code so far.

Thanks,

Mathieu

> 
> -- 
> Arjan van de Ven 	Intel Open Source Technology Centre
> For development, discussion and tips for power savings, 
> visit http://www.lesswatts.org

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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