linux-kernel - Re: [PATCH] scripts/recordmcount.pl: Support build with -ffunction-sections.

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Message-ID: <1351613128.8467.109.camel@gandalf.local.home>
Date:	Tue, 30 Oct 2012 12:05:28 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Will Newton <will.newton@...il.com>
Cc:	Linux Kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] scripts/recordmcount.pl: Support build with
 -ffunction-sections.

On Tue, 2012-10-30 at 15:35 +0000, Will Newton wrote:
> On Tue, Oct 30, 2012 at 3:05 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> > On Tue, 2012-10-30 at 14:51 +0000, Will Newton wrote:
> >> Scan any text section whose name begins with ".text." so
> >> we will find all the functions in a kernel built with
> >> -ffunction-sections.
> >
> > A couple of things.
> >
> > First, I'm very paranoid about a blanket "ok" on sections. We must
> > guarantee that all sections that starts with ".text" never is freed. And
> > if it is freed, that it must inform ftrace that it's about to free it
> > before it does. If we can not guarantee this, then we can't do it.
> 
> I share that concern, however it looks like it should be safe. 

Can we confirm that it is safe. I'm not too confident with the words
"looks like" ;-)


> The two
> architectures in tree that enable -ffunction-sections by default are
> parisc and score and they both use a .text.* wildcard in their linker
> script. If the support for building with --gc-sections is ever
> completed then that would require -ffunction-sections and a similar
> wildcard to be applied to section names.

We need to make a way to guarantee any section that is added as ".text"
is never removed (freed), except for modules, which already have a way
to notify ftrace that its removing its text.

> 
> > Second, most archs today do not use recordmcount.pl. They use the new
> > recordmcount.c file.
> 
> Ok, thanks for letting me know. Is the plan to switch over to
> recordmcount.c exclusively at some point?

When all the archs convert.

> 
> The following architectures have some support in recordmcount.c but do
> not enable it:
> 
> sh, powerpc, ia64

Hmm, I have a powerpc, I should get that fixed.

> 
> And the following have no support in recordmcount.c as yet:
> 
> blackfin, microblaze

It's up to the arch maintainers to do the work. If the perl script is
good enough for them then I have no problem maintaining both. But I
don't have the machines to test these archs.

-- Steve


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