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

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Message-ID: <c62985530810301130q41cc5c0ete2d7610d2172775@mail.gmail.com>
Date:	Thu, 30 Oct 2008 19:30:20 +0100
From:	"Frédéric Weisbecker" <fweisbec@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	"Steven Rostedt" <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] Make ftrace able to trace function return

2008/10/30 Ingo Molnar <mingo@...e.hu>:
> hm, are you aware of the -finstrument-functions feature of GCC?
>
> that feature generates such entry points at build time:
>
>                   void __cyg_profile_func_enter (void *this_fn,
>                                                  void *call_site);
>                   void __cyg_profile_func_exit  (void *this_fn,
>                                                  void *call_site);

Oh I didn't know it.
But wouldn't it conflict with the -pg used for mcount in ftrace?

> this might be faster/cleaner than using a trampoline approach IMO.

Yes, oubviously!

> OTOH, entry+exit profiling has about double the cost of just entry
> profiling - so maybe there should be some runtime flexibility there.
> Plus the same recordmcount trick should be used to patch up these
> entry points to NOP by default.

Yes that's right. Hm I don't know what to do since there is already mcount....
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