linux-kernel - Re: [RFC PATCH 1/4] ftrace: Make recordmcount.c handle __fentry_

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Date:	Tue, 07 Aug 2012 16:57:58 -0700
From:	John Reiser <jreiser@...wagon.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [RFC PATCH 1/4] ftrace: Make recordmcount.c handle __fentry__

On 08/07/2012 12:38 PM, Steven Rostedt wrote:
> From: Steven Rostedt <srostedt@...hat.com>
> 
> With gcc 4.6.0 the -mfentry feature places the function profiling call at the start of the function. When this is used, the call is to __fentry__ and not mcount.
> 
> Change recordmcount.c to record both callers to __fentry__ and mcount.
   [snip]
> -	    (altmcount && strcmp(altmcount, symname) == 0))
> +	    (altmcount && strcmp(altmcount, symname) == 0) ||
> +	    (strcmp(fentry, symname) == 0))


The proposed change will work as long as all the *.o use the same name.
Only one of {"__fentry__", "mcount", "_mcount", altmcount} is allowed
for all the *.o as input for a particular run.  [Modulo the hack
of ignoring a leading '.' for 64-bit PowerPC, of course.]

If the user changes compilers (or changes CFLAGS by insert/remove "-mfentry")
without doing a "make clean", then recordmcount will omit some calls.

Those restrictions are easy to guess, and recovery is easy.  Therefore,
Ack'ed by: John Reiser <jreiser@...wagon.com>

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