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

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Date:	Wed, 20 Sep 2006 12:22:02 -0700
From:	Martin Bligh <mbligh@...gle.com>
To:	karim@...rsys.com
CC:	"Frank Ch. Eigler" <fche@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	prasanna@...ibm.com, Andrew Morton <akpm@...l.org>,
	Ingo Molnar <mingo@...e.hu>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Paul Mundt <lethal@...ux-sh.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Jes Sorensen <jes@....com>, 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

Karim Yaghmour wrote:
> Martin Bligh wrote:
> 
>>It's looking to me like it might still need djprobes to implement, in
>>order to get the atomic and safe switchover from the original function
>>into the traced one. All rather sad, but seems to be true from all the
>>CPU errata, etc. If anyone can see a way round that, I'd love to hear
>>it.
> 
> 
> But we don't need to fight the errata, there are fortunately solutions
> that take care of it where it does exist (x86: djprobes/kprobes.)
> What's more interesting, though, is that the method as it is proposed
> at this stage *seems* to be easily portable to other archs. And where
> such binary trickery is difficult to pull off, nothing precludes
> having a universally "portable" mechanism including something akin to
> switching between instrumented vs. normal function at function entry.
> Even such conditional ifs can be optimized by the CPU nowadays.
> 
> The picture is, nevertheless, very bright at the moment (I think).
> Just have a 5byte filler at function entry such as Hiramatsu-san
> suggested, and use djprobes to fork to instrumented function. The
> unconditional jump in the filler will most likely be utterly
> unmeasurable, and benchmarks should confirm this.
> 
> So:
> On x86: use 5byte filler and djprobes.
> On "sane" archs: use filler and override as explained earlier.
> Elsewhere: use standard "if" or function pointer at function entry.

Do we even need the filler padding? I thought we could insert kprobes
at the beginning of any function without that ... it was only a
requirement for mid-function (sometimes). If we copy the whole function,
we don't even need that any more ...

if kprobes can do it, I don't see why djprobes can't ... after all, it
just seems to use kprobes to insert a jump, AFAICS.

M.

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