linux-kernel - Re: MIPS Function Tracer question

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Date:	Mon, 03 Dec 2012 09:40:48 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	David Daney <ddaney.cavm@...il.com>
Cc:	Alan Cooper <alcooperx@...il.com>, ralf@...ux-mips.org,
	linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org
Subject: Re: MIPS Function Tracer question

On Thu, 2012-11-29 at 15:00 -0800, David Daney wrote:
> On 11/29/2012 01:04 PM, Alan Cooper wrote:
> > I've been doing some testing of the MIPS Function Tracer functionality
> > on the 3.3 kernel. I was surprised to find that the option to generate
> > frame pointers was required for tracing.
> 
> It is not really required for MIPS function tracing, but the Kconfigs 
> for some reason set it.
> 

The issue is with x86. Gcc wont compile if you have -pg and
-fomit-frame-pointer on x86. I originally forced function tracing to
select FRAME_POINTER, but because now on x86 with -mfentry, -pg no
longer requires frame pointers being set, I just let -pg complain one
way or the other. I believe that gcc by default will not add frame
pointers. Thus adding function tracing just prevents
-fomit-frame-pointer from being set, and if -pg requires frame pointers
it will automatically enable them otherwise they should not be enabled.

-- Steve

> >  When I don't enable
> > FRAME_POINTER along with FUNCTION_TRACER, the kernel hangs on boot. I
> > also noticed that a checkin to the 3.4 kernel
> > (b732d439cb43336cd6d7e804ecb2c81193ef63b0) no longer forces on
> > FRAME_POINTER when FUNCTION_TRACER is selected. I was wondering how it
> > works in 3.4 and beyond, so I built a Malta kernel from the latest
> > MIPS tree with FUNCTION_TRACING enabled and tested it with QEMU. The
> > kernel hung the same way. I can think of 2 reasons for this:
> > 1. Function tracing is broken for MIPS in 3.4 and beyond.
> > 2. The 4.5.3 GNU C compiler I'm using is generating different code for
> > function tracing.
> 


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