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

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Date:	Fri, 15 Sep 2006 10:57:29 -0400
From:	Karim Yaghmour <karim@...rsys.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	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>,
	Andrew Morton <akpm@...l.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


Alan Cox wrote:
> The gdb debug data lets you find each line and also the variable
> assignments (except when highly optimised in some cases). Try
> breakpointing there with kgdb and using "where"... A kgdb script is the
> wrong way to do instrumentation but it does demonstrate the information
> is already out there, automatically generated and self maintaining.
> 
> You do need the gdb -g debug data, but equally if it was static you'd
> need to recompile with the tracepoint because it would be off by
> default, and there is a very small risk in both cases you'll disturb or
> change the code behaviour/flow.
...
> Thats why we have things like systemtap.
> 
> All we appear to lack is systemtap ability to parse debug data so it can
> be told "trace on line 9 of sched.c and record rq and next"

Thanks for the explanation. But I submit to you that both explanations
actually highlight the argument I was making earlier with regards to
dynamic tracing (and gdb info in this case) actually require a non-
expert to chase kernel versions and create appropriate appropriate
scripts/config-info for the post-insertion of instrumentation, with
the risks to kernel developers this may have (ex.: bug report to
lkml from user claiming to have discovered problem in subsystem when,
in fact, trace point by external maintainer was ill-chosen.)

Cheers,

Karim
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