linux-kernel - Re: [PATCH 2/3] ftrace/x86: Add support for C version of recordmcount

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Date:	Fri, 15 Oct 2010 05:18:32 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-arch@...r.kernel.org, Michal Marek <mmarek@...e.cz>,
	linux-kbuild@...r.kernel.org, John Reiser <jreiser@...wagon.com>
Subject: Re: [PATCH 2/3] ftrace/x86: Add support for C version of recordmcount


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

> On Fri, 2010-10-15 at 04:50 +0200, Ingo Molnar wrote:
> > * Steven Rostedt <rostedt@...dmis.org> wrote:
> > 
> > > From: Steven Rostedt <srostedt@...hat.com>
> > > 
> > > This patch adds the support for the C version of recordmcount and
> > > compile times show ~ 12% improvement.
> > 
> > I reported this recordmcount performance problem 2 years ago. Better 
> > later than never i guess.
> 
> And I also remember saying after I posted this code that it would have 
> a compile time performance hit. Heck, it's a perl script running on 
> every object file. It was obvious what was at issue here. But it's 
> better to slow down the kernel build than to brick network cards. 

Well, it's even better to do neither!

> Also, perl was much easier to do.

Lets write the whole kernel in perl and forget about performance ;-)

> That said, the embarrassing thing is not that I knew (or you reported 
> it) about this performance problem. I'm actually quite embarrassed 
> that I had this code sitting in my inbox for over a year. I just kept 
> having other things that were more important coming up than lowering 
> the compile time of the kernel. Although, I did work to get streamline 
> config to offset this performance hit.
> 
> Finally, while at the End Users Summit, I decided to take a look at 
> John's code, and I was quite impressed.
> 
> But as you said, better late than never.

Yeah. Note that as a maintainer i need to grumble when i see some 
not-so-good event - even if there's a happy resolution! Otherwise such 
cases would tend to creep up in frequency ;-)

> > > +ifdef CONFIG_DYNAMIC_FTRACE
> > > +	ifdef CONFIG_HAVE_C_MCOUNT_RECORD
> > > +		BUILD_C_RECORDMCOUNT := y
> > > +		export BUILD_C_RECORDMCOUNT
> > 
> > > --- a/arch/x86/Kconfig
> > > +++ b/arch/x86/Kconfig
> > > @@ -33,6 +33,7 @@ config X86
> > >  	select HAVE_KRETPROBES
> > >  	select HAVE_OPTPROBES
> > >  	select HAVE_FTRACE_MCOUNT_RECORD
> > > +	select HAVE_C_MCOUNT_RECORD
> > 
> > The naming is inconsistent here - it should be HAVE_C_RECORDMCOUNT, like 
> > the build variable has, and like the utility is called. If we are going 
> > to add this flag to most architectures we should name it consistently.
> 
> Sure, want me to rebase it or just write a patch on top of it?

Sure, patch on top would be fine.

Thanks,

	Ingo
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