linux-kernel - Re: [PATCH 000/208] big x86 FPU code rewrite

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Date:	Tue, 5 May 2015 19:50:58 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 000/208] big x86 FPU code rewrite


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Tue, May 5, 2015 at 9:23 AM, Ingo Molnar <mingo@...nel.org> wrote:
> >  83 files changed, 3742 insertions(+), 2841 deletions(-)
> 
> How much of this is just the added instrumentation? [...]

Half of it is that, plus a lot of comments.

> [...] Because that's almost a thousand new lines, which makes me 
> unhappy. The *last* thing we want is to make this thing bigger. 
> [...]

So Boris suggested that I should move fpu/measure.c out of the FPU 
code anyway, which is fair enough, as it measures a lot of other low 
level details as well. Consider it done.

With that taken out, the diffstat comes down to:

   81 files changed, 3409 insertions(+), 3055 deletions(-)

That's mostly 400 new lines of comments all around the FPU code, plus 
a bit of extra headers due to the split-up modules (50-100 lines 
maybe).

> [...] I'm not convinced it's worth it adding some performance debug 
> code that doesn't really add any new information, and could be done 
> outside the kernel as just an independent module instead.

Code size difference (with debugging off) on an x86-64 defconfig-ish 
kernel:

        text      data    bss     filename

    15030376   2574976   1634304 vmlinux.before
    15023690   2578648   1634304 vmlinux.after

The runtime size of the kernel got smaller by 7K.

Considering that arch/x86/kernel/fpu/built-in.o is only 13K that's 
quite significant.

Thanks,

	Ingo
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