Re: [PATCH] kbuild: pass jobserver to cmd_ld_vmlinux.o

[Nathan Chancellor <nathan@kernel.org>]

On Fri, Jun 17, 2022 at 07:50:58PM +0200, Sedat Dilek wrote:
> On Fri, Jun 17, 2022 at 6:32 PM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
> >
> > On Fri, Jun 17, 2022 at 3:35 AM Sedat Dilek <sedat.dilek@gmail.com> wrote:
> > >
> > > On Fri, Jun 17, 2022 at 12:53 AM Sedat Dilek <sedat.dilek@gmail.com> wrote:
> > > >
> > > > On Thu, Jun 16, 2022 at 4:09 PM Sedat Dilek <sedat.dilek@gmail.com> wrote:
> > > > >
> > > > > On Thu, Jun 16, 2022 at 12:45 PM Jiri Slaby <jslaby@suse.cz> wrote:
> > > > > >
> > > > > > Until the link-vmlinux.sh split (cf. the commit below), the linker was
> > > > > > run with jobserver set in MAKEFLAGS. After the split, the command in
> > > > > > Makefile.vmlinux_o is not prefixed by "+" anymore, so this information
> > > > > > is lost.
> > > > > >
> > > > > > Restore it as linkers working in parallel (esp. the LTO ones) make a use
> > > > > > of it.
> >
> > TBH, I agree with Masahiro. I didn't understand this comment "esp. the
> > LTO ones."  Jiri, can you clarify what you mean here?
> >
> > > [ CC Nathan and Sami ]
> > >
> > > I re-checked as my build-time takes approx. 15mins more than usual
> > > with Linux v5.17.
> > >
> > > [ Perf - seconds time elapsed ]
> > > v5.17: approx. 12500
> > > v5.19: approx. 13500
> > >
> > > +1.000 secs = +16,67mins

So roughly 7.5% increase if I did my math correctly?

> > Does this patch make a difference?
> > https://lore.kernel.org/linux-kbuild/20220616195759.3214538-1-samitolvanen@google.com/
> >
> 
> Running a new build...

I did a quick benchmark with Fedora's clang 14.0.0 and I didn't see an
increase of that proportion with your configuration (only about 1%,
which I would expect would be within the noise for new drivers and
such):

Benchmark 1: 5.17
  Time (abs ≡):        287.744 s               [User: 23497.818 s, System: 4444.602 s]

Benchmark 2: 5.19-rc2
  Time (abs ≡):        290.027 s               [User: 24567.290 s, System: 4834.458 s]

Summary
  '5.17' ran
    1.01 times faster than '5.19-rc2'

Not to say that there is not an issue but it might be more visible with
slower hardware.

Cheers,
Nathan