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

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Message-ID: <20060915181208.GA17581@elte.hu>
Date:	Fri, 15 Sep 2006 20:12:08 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	"Frank Ch. Eigler" <fche@...hat.com>, karim@...rsys.com,
	Roman Zippel <zippel@...ux-m68k.org>,
	Tim Bird <tim.bird@...sony.com>,
	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 <alan@...rguk.ukuu.org.uk> wrote:

> Ar Gwe, 2006-09-15 am 13:08 -0400, ysgrifennodd Frank Ch. Eigler:
> > Alan Cox <alan@...rguk.ukuu.org.uk> writes:
> > - where 1000-cycle int3-dispatching overheads too high
> 
> Why are your despatching overheads 1000 cycles ? (and if its due to 
> int3 why are you using int 3 8))

this is being worked on actively: there's the "djprobes" patchset, which 
includes a simplified disassembler to analyze common target code and can 
thus insert much faster, call-a-trampoline-function based tracepoints 
that are just as fast as (or faster than) compile-time, static 
tracepoints.

there's no fundamental reason why INT3 should be the primary model of 
inserting kprobes. Sometimes we are unlucky and the code which we target 
is too complex - then we take a few hundred cycles of a penalty. If that 
piece of code is a really common destination then we can add a static 
marker in the source which both prepares parameters and inserts a 
sufficiently sized NOP (or a function call) to prepare things for fast 
dynamic tracing - but it should only be an optional performance helper 
that we have the freedom to zap.

(kprobes can be thought of as a special "JIT", and there's no 
fundamental reason why it couldnt do almost arbitrary transformations on 
kernel code.)

and there's alot more that kprobes/systemtap can do: it can be a method 
of extending the kernel along a 'plugin' model - without having to 
impact the kernel source! That way people can experiment with kernel 
extensions on live kernels, without the barrier of recompile/reboot.

	Ingo
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