linux-kernel - Re: [PATCH 0/11] LTTng-core (basic tracing infrastructure) 0.5.108

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Date:	Fri, 15 Sep 2006 12:59:34 -0700
From:	Andrew Morton <akpm@...l.org>
To:	karim@...rsys.com
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Roman Zippel <zippel@...ux-m68k.org>,
	Tim Bird <tim.bird@...sony.com>, Ingo Molnar <mingo@...e.hu>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tom Zanussi <zanussi@...ibm.com>, ltt-dev@...fik.org,
	Michel Dagenais <michel.dagenais@...ymtl.ca>
Subject: Re: [PATCH 0/11] LTTng-core (basic tracing infrastructure) 0.5.108

On Fri, 15 Sep 2006 14:16:18 -0400
Karim Yaghmour <karim@...rsys.com> wrote:

> > Although IMO this is a bit lame - it is quite possible to go into
> > SexySystemTapGUI, click on a particular kernel file-n-line and have
> > systemtap userspace keep track of that place in the kernel source across
> > many kernel versions: all it needs to do is to remember the file+line and a
> > snippet of the surrounding text, for readjustment purposes.
> 
> Sure, if you're a kernel developer, but as I've explained numberous
> times in this thread, there are far more many users of tracing than
> kernel developers.

Disagree.  I was describing a means by which a set of systemtap trace
points could be described.  A means which would allow those tracepoints to
be maintained without human intervention as the kernel source changes. 
(ie: use a similar algorithm and representation as patch(1)).

Presumably those tracepoints would have been provided by a kernel developer
and delivered to non-developers, just like static tracepoints.

> > (*) I don't buy the performance arguments: kprobes are quick, and I'd
> > expect that the CPU consumption of the destination of the probe is
> > comparable to or higher than the cost of taking the initial trap.
> 
> Please see Mathieu's earlier posting of numbers comparing kprobes to
> static points. Nevertheless, I do not believe that the use of kprobes
> should be pitted against static instrumentation, the two are
> orthogonal.

People have been speeding up kprobes in recent kernels, to avoid the int3
overhead.  I don't recall seeing how effective that has been.
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