linux-kernel - Re: [PATCH 3/9 - v2][RFC] tracing: Remove per event trace registering

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Date:	Fri, 7 May 2010 16:58:56 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Christoph Hellwig <hch@....de>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 3/9 - v2][RFC] tracing: Remove per event trace
	registering

* Steven Rostedt (rostedt@...dmis.org) wrote:
> On Fri, 2010-05-07 at 20:01 +0200, Frederic Weisbecker wrote:
> > On Fri, May 07, 2010 at 10:54:38AM -0400, Mathieu Desnoyers wrote:
> 
> > The second is this extra parameter passed whether or not it is needed.
> > And although we suppose it is safe, I don't feel comfortable with it.
> > So if we can find a more proper way to avoid it, I'm all for it.
> 
> Now I'm making the extra parameter mandatory for all tracepoint
> probes. ;-)
> 
> But this time, it will be at the start not the end.
> 
> 	void probe(void *data, proto);
> 
> 
> Unfortunately we can't avoid it. In order to remove the extra code
> (registering and unregistering) and even share the probe among several
> events, we need a way to pass the data to the probe to let the probe
> know what event it is dealing with (to put in the event id into the
> buffer, to let the tracer output code know what event this data is for).
> 
> The current method is that only the proto that the tracepoint uses is
> passed to the probe. This gives us no way to add any more information.
> 
> This new method allows data to be assigned at probe register, and the
> probe gets this data as the first parameter.
> 
> The register_* functions will still do typechecking of the probes, they
> just add the "void *" at the beginning.
> 
> Actually, here is a place that I can see where Mathieu's check does come
> in handy. If we add the check test to each probe, and the tracepoint
> proto changes, it will flag it.

Ah ! finally we are getting on the same page. :-)

> 
> Mathieu, you've been explaining this wrong ;-)

Yep, it seems like I've done a terrible job at trying to explain my motivation
to you. ;)

> 
> I'm not worried about changes to ftrace.h breaking things. I'm worried
> about changes to tracepoint.h breaking ftrace.h.  This is where your
> check comes in. As I change the void *data from the end to the start,
> I'm nervous about catching all the probes that are registered this way.
> (ftrace events, syscalls, kprobes, and perf)

I'm nervous about catching all tracepoint-related problems that may arise from
both tracepoint and tracepoint probes prototype changes, yes.

I'm glad we agree then. :-)

Thanks,

Mathieu


> 
> -- Steve
> 
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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