linux-kernel - Re: [RFC][PATCH] Make ftrace able to trace function return

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Date:	Thu, 30 Oct 2008 20:26:21 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frédéric Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] Make ftrace able to trace function return

* Steven Rostedt <rostedt@...dmis.org> wrote:

> gcc -c -finstrument-functions traceme.c

> This is not 5 extra bytes but 27 extra bytes for a total of 32 bytes 
> at every function. Also note that this also adds these calls to 
> inline functions as well. We could easly stop that by adding 
> "notrace" to the inline define (which I've done).
> 
> But this would make the patching a bit more difficult (not 
> impossible). But it will bloat the image quite a bit.

yeah, i too knew it was expensive, but didnt realize it's this bloaty. 
It's a non-starter really.

How reliable can we make Frederic's trampoline approach? Especially 
with the opt-in function filters of dyn-ftrace, there's real value in 
the trampoline approach IMO (we could display real function cost, 
etc.), and the runtime cost should be OK as long we dont probe a ton 
of functions all the time. (and it's optional anyway)

	Ingo
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