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

Ingo Molnar
2015-05-05T17:51:07Z

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

> On Tue, May 5, 2015 at 9:23 AM, Ingo Molnar <mingo@kernel.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