linux-kernel - Re: [PATCH] Linux Kernel Markers

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Date:	Tue, 19 Sep 2006 17:31:07 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Martin J. Bligh" <mbligh@...gle.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Jes Sorensen <jes@....com>, Andrew Morton <akpm@...l.org>,
	Tom Zanussi <zanussi@...ibm.com>,
	Richard J Moore <richardj_moore@...ibm.com>,
	Michel Dagenais <michel.dagenais@...ymtl.ca>,
	Christoph Hellwig <hch@...radead.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	William Cohen <wcohen@...hat.com>, ltt-dev@...fik.org,
	systemtap@...rces.redhat.com, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] Linux Kernel Markers


* Martin J. Bligh <mbligh@...gle.com> wrote:

> You know ... it strikes me that there's another way to do this, that's 
> zero overhead when not enabled, and gets rid of the inflexibility in 
> kprobes. It might not work well in all cases, but at least for simple 
> non-inlined functions, it'd seem to.
> 
> Why don't we just copy the whole damned function somewhere else, and 
> make an instrumented copy (as a kernel module)? Then reroute all the 
> function calls through it, instead of the original version. OK, it's 
> not completely trivial to do, but simpler than kprobes (probably doing 
> the switchover atomically is the hard part, but not impossible). 
> There's NO overhead when not using, and much lower than probes when 
> you are.
> 
> That way we can do whatever the hell we please with internal 
> variables, however GCC optimises it, can write flexible instrumenting 
> code to just about anything, program in C as God intended, etc, etc. 
> No, it probably won't fix every case under the sun, but hopefully most 
> of them, and we can still use kprobes/djprobes/bodilyprobes for the 
> rest of the cases.

yeah, this would be nice - if it werent it for function pointers, and if 
all kernel functions were relocatable. But if you can think of a method 
to do this, it would be nice.

	Ingo
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