Re: [PATCH] arch/x86: Add an option to build the kernel with '-march=native' on x86-64

[Tor Vic]

On 3/22/25 12:40, Arnd Bergmann wrote:
> On Fri, Mar 21, 2025, at 15:28, Tor Vic wrote:
> > Add a 'native' option that allows users to build an optimized kernel for
> > their local machine (i.e. the machine which is used to build the kernel)
> > by passing '-march=native' to the CFLAGS.
> >
> > The idea comes from Linus' reply to Arnd's initial proposal in [1].
> >
> > This patch is based on Arnd's x86 cleanup series, which is now in -tip [2].
>
> Thanks for having another look at this and for including the
> benchmarks. I ended up dropping this bit of my series because
> there were too many open questions around things like
> reproducible builds, but there is clearly a demand for having
> this included.
>
> >        hackbench (lower is better):
> >        102.27 --> 99.50                         (-2.709 %)
> >
> >    - stress-ng, bogoops, average of 3 15-second runs:
> >        fork:
> >        111'744 --> 115'509                      (+3.397 %)
> >        bsearch:
> >        7'211 --> 7'436                          (+3.120 %)
> >        vm:
> >        1'442'256 --> 1'486'615                  (+3.076 %)
>
> 3% in userspace benchmarks does seem significant enough to
> spend more time on seeing what exactly made the difference
> here, and possibly including it as separate options.
>
> > +ifdef CONFIG_NATIVE_CPU
> > +        KBUILD_CFLAGS += -march=native
> > +        KBUILD_RUSTFLAGS += -Ctarget-cpu=native
> > +else
> >           KBUILD_CFLAGS += -march=x86-64 -mtune=generic
> >           KBUILD_RUSTFLAGS += -Ctarget-cpu=x86-64 -Ztune-cpu=generic
> > +endif
>
> I assume that the difference here is that -march=native on
> your machine gets turned into -march=skylake, which then turns
> on both additional instructions and a different instruction
> scheduler.
>
> Are you able to quickly run the same tests again using
> just one of the two?
>
> a) -march=x86-64 -mtune=skylake
> b) -march=skylake -mtune=generic

I ran the tests on Zen2 (znver2), but this time the kernel was built 
with clang-20+lto (the skylake tests were with gcc-14).

It turned out that with '-march=native', there is almost no difference 
compared to '-march=x86-64'.
All results are within +0.8% and -0.6%, most of which are probably 
noise. Hackbench, stress-ng fork and xz compression seem to profit the 
most from 'native'.

The vmlinux image is 0.03% bigger with 'native'.

I guess that 'native' can be somewhat useful on some architectures, but 
not on all...

As for your question to run with both '-march' and '-mtune', I'm sorry, 
but I didn't have the time just yet.


>      Arnd