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

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Message-ID: <4510175B.7000200@google.com>
Date:	Tue, 19 Sep 2006 09:14:19 -0700
From:	Martin Bligh <mbligh@...gle.com>
To:	Vara Prasad <prasadav@...ibm.com>
CC:	Ingo Molnar <mingo@...e.hu>,
	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

> It is an interesting idea but there appears to be following hard issues 
> (some of which you have already listed) i am not able to see how we can 
> overcome them
> 
> 1) We are going to have a duplicate of the whole function which means 
> any significant changes in the original function needs to be done on the 
> copy as well, you think maintainers would like this double work idea.

No, no ... the duplicate function isn't duplicated source code, only
object code. Either a config option via the markup macros that we've
been discussing, or something I hack up on the fly to debug a problem
dynamically. In terms of how the debugging-type source code is kept,
it's no different than something like systemtap or LTT (either would
work, and a normal diff could be used to keep out of tree stuff),
it's just how it hooks in is different to kprobes.

> 2) Inline functions is often the place where we need a fast path to 
> overcome the current kprobes overhead.

You can still instrument inline functions, you just need to hook all
the callers, not the inline itself.

> 3) As you said it is not trivial across all the platforms to do a switch 
> to the instrumented function from the original during the execution.  
> This problem is similar to the issue we are dealing with djprobes.

If we just freeze all kernel operations for a split second whilst we do
this, does it matter? Or even if we don't ... there's a brief race where
some calls are traced, and some are not ... does that even matter?
Doesn't seem like most usages would care.

M.

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