linux-kernel - Re: [PATCH] Linux Kernel Markers 0.2 for Linux 2.6.17

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Message-ID: <20060920155358.GH18646@redhat.com>
Date:	Wed, 20 Sep 2006 11:53:58 -0400
From:	"Frank Ch. Eigler" <fche@...hat.com>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
Cc:	Karim Yaghmour <karim@...rsys.com>, 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>,
	Douglas Niehaus <niehaus@...s.ku.edu>,
	Tom Zanussi <zanussi@...ibm.com>,
	Paul Mundt <lethal@...ux-sh.org>, Jes Sorensen <jes@....com>,
	Richard J Moore <richardj_moore@...ibm.com>,
	William Cohen <wcohen@...hat.com>,
	"Martin J. Bligh" <mbligh@...igh.org>,
	Michel Dagenais <michel.dagenais@...ymtl.ca>,
	systemtap@...rces.redhat.com, ltt-dev@...fik.org
Subject: Re: [PATCH] Linux Kernel Markers 0.2 for Linux 2.6.17

Hi -

Mathieu Desnoyers wrote:

> [...]  Do you have ideas on how we can export the function symbol?
> (is it necessary ?)

It turns out that static variables like that get included in the
ordinary symbol tables along with other (un)initialized globals - it
has been making it into /proc/kallsyms.  If the normal symbol table is
not available, then some other measure would be needed to find the
variable containing the function pointer.

> [...]
> #define MARK(name, format, args...) \
>         do { \
>                 __mark_check_format(format, ## args); \
>                 MARK_SYM(name); \
>                 MARK_CALL(name, format, ## args); \
>         } while(0)

While varargs simplify some things, it sacrifices type-safety, in that
a handler function would have to be varargs too.  For the systemtap
marker prototype, parametrized variants use scores of (automatically
generated) macros, with different arity/type permutations, each
self-describing and type-safe.

Regarding a marker variant that would require kprobes (inserting a
labelled NOP or few), it may be an appropriate choice where dormant
marker overhead must be minimal and robust parameter passing is less
important.

- FChE

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