linux-kernel - Re: [PATCH] Linux Kernel Markers 0.11 for 2.6.17

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Date:	26 Sep 2006 12:39:38 -0400
From:	fche@...hat.com (Frank Ch. Eigler)
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
Cc:	Martin Bligh <mbligh@...gle.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	prasanna@...ibm.com, Andrew Morton <akpm@...l.org>,
	Ingo Molnar <mingo@...e.hu>, 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>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Karim Yaghmour <karim@...rsys.com>,
	Pavel Machek <pavel@...e.cz>, Joe Perches <joe@...ches.com>,
	"Randy.Dunlap" <rdunlap@...otime.net>,
	"Jose R. Santos" <jrs@...ibm.com>
Subject: Re: [PATCH] Linux Kernel Markers 0.11 for 2.6.17

Mathieu Desnoyers <compudj@...stal.dyndns.org> writes:

> [...]
> > I believe [printf formatting directives] are not
> > quite general enough either e.g. to describe a raw binary blob.
>
> If you want to dump a raw binary blob, what about :
> MARK(mysubsys_myevent, "char %p %u", blobptr, blobsize); where %p is
> a pointer to an array of char and %u the length ?

That involves new conventions beyond printf.  Why not "%p %p %u %u"
for two blobs ... or why implicitly dereference the given pointers.  A
probe handler unaware of a specific marker's semantics would not know
whether or not this is implied.


> My idea is to use the string to identify what is referred by a
> pointer, so it can be casted into this type with some kind of
> coherency between the marker and the probe.

I understand what you're using them for.  To me, they just don't look
like a good fit.


> > I realize they serve a useful purpose in abbreviating what otherwise
> > one might have to do (like that multiplicity of STAP_MARK_* type/arity
> > permutations).  [...]
> 
> I think that duplicating the number of marker macros could easily make
> them unflexible and ugly. [...]

Inflexible and ugly in what way?  Remember, the macro definitions can
be automatically generated.  At the macro call site, there needs to be
little difference.


> [...]  Good point, I will setup a va_args in the probe.  When
> correctly used, however, there is no need to use the format string :
> we can directly get the variables from the var arg list if we know
> in advance what the string will be.

Do I understand you correctly that the probe handlers would be given
va_list values, and would have to call va_arg to yank out individual
actual arguments?  So again type safety is a matter of explicit coding
(equivalent to correctly casting each type)?


- FChE
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