Re: [GIT PULL] Kbuild fixes for v6.15-rc7

Re: [GIT PULL] Kbuild fixes for v6.15-rc7

[Xose Vazquez Perez]

Linus Torvalds wrote:

> There's probably a small amount of "the kernel keeps growing, so build
> inevitably slow down anyway", but yeah, it's probably just the slower
> compiler that is what I notice.

gcc-14 is also available:

$ dnf install gcc14
$ rpm -ql gcc14 | grep bin
/usr/bin/cpp-14
/usr/bin/gcc-14
/usr/bin/gcc-ar-14
/usr/bin/gcc-nm-14
/usr/bin/gcc-ranlib-14
/usr/bin/gcov-14
/usr/bin/gcov-dump-14
/usr/bin/gcov-tool-14
/usr/bin/lto-dump-14
/usr/bin/x86_64-redhat-linux-gcc-14

Re: [GIT PULL] Kbuild fixes for v6.15-rc7

[Linus Torvalds]

On Mon, 26 May 2025 at 23:28, Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> However, building the kernel in Fedora 42 is 30% slower
> than in Fedora 41.
>
> So, maybe, this is because you recently upgraded your OS to Fedora 42.

Yeah, so that explains all - or at least most - of it.

There's probably a small amount of "the kernel keeps growing, so build
inevitably slow down anyway", but yeah, it's probably just the slower
compiler that is what I notice.

Thanks for checking for me,

               Linus

Re: [GIT PULL] Kbuild fixes for v6.15-rc7

[Masahiro Yamada]

Hi Linus,

On Tue, May 27, 2025 at 3:16 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> Masahiro,
>
> On Wed, 14 May 2025 at 22:15, Masahiro Yamada <masahiroy@kernel.org> wrote:
> >
> > Please pull some Kbuild fixes.
>
> So this isn't actually about your latest kbuild fixes, but more a
> generic kbuild thing now that the merge window has opened..
>
> My build times have gone up by a *lot* lately (and by "lately" I don't
> mean very recently, but in the last month or so), to the point that
> I'm now finding it to be a real annoyance due to it slowing down the
> merge window for me.
>
> I did a profile on the load just to check, and it's all just gcc
> (well, cc1 mainly), so it's not that something else like ccache got
> enabled.
>
> (Things like ccache can speed up builds in other scenarios, but I've
> also seen it slow things down a lot for the worst-case sitation when
> header files change, so I wanted to verify that it's not just that)
>
> So it's just that we're spending a lot more time in the compiler.
>
> I haven't tried to bisect it, and now I'm too busy to, but the two
> main suspects are either that KASAN is now enabled and that just makes
> the builds a lot slower (if so, it's my own fault - introduced by me
> in commit 6f110a5e4f99 "Disable SLUB_TINY for build testing").
>
> Or it's just that I upgraded to F42 and gcc-15 itself is perhaps just
> that much slower.
>
> Anyway, I'm emailing you exactly because I'm busy with the merge
> window and this is causing issues for me but I don't have time to
> figure it out - and so I'm going "maybe Masahiro is aware of this
> already"

I am not aware of the slowness you mentioned.

I compared v6.15-rc1 and v6.15-1972-g914873bc7df9
(today's latest), but I did not see noticable slowness.

masahiro@oscar:~/ref/linux((HEAD detached at v6.15-rc1))$ git describe
v6.15-rc1
masahiro@oscar:~/ref/linux((HEAD detached at v6.15-rc1))$ git clean
-fdx && time make -j24 defconfig all
 [ snip ]
real 1m46.444s
user 27m59.363s
sys 4m47.092s


masahiro@oscar:~/ref/linux(master)$ git describe
v6.15-1972-g914873bc7df9
masahiro@oscar:~/ref/linux(master)$ git clean -fdx && time make -j24
defconfig all
[snip]
real 1m45.410s
user 28m11.197s
sys 4m45.550s


However, building the kernel in Fedora 42 is 30% slower
than in Fedora 41.

So, maybe, this is because you recently upgraded your OS to Fedora 42.




[In Fedora 41 Docker container]

[masahiro@495de5a516cc linux]$ gcc --version | head -n1
gcc (GCC) 14.3.1 20250523 (Red Hat 14.3.1-1)
[masahiro@495de5a516cc linux]$ git clean -fdx && time make -j24 defconfig all
  [snip]
 real 2m25.794s
user 25m18.131s
sys 5m56.074s



[In Fedora 42 Docker container]

[masahiro@a55a26dda811 linux]$ gcc --version | head -n1
gcc (GCC) 15.1.1 20250521 (Red Hat 15.1.1-2)
[masahiro@a55a26dda811 linux]$ git clean -fdx && time make -j24 defconfig all
  [snip]
real 3m12.714s
user 35m59.299s
sys 9m10.699s







> I also did notice while timing things that the final module link time
> seems to have limited threading
>
> For the rest of the build, I have load averages in the 100+ range
> (because I build with "-j128"), but then the last few minutes it
> starts dropping a lot when it's in the
>
>   CC [M] ...
>
> stage, and when it hits that Makefile.modfinal stage and we get
>
>   LD [M] ...
>
> stage it seems to have serialized completely and just spends 100% of
> time in one single 'make' instance.
>
> But that may be an effect of the link being so fast that 'ps' and
> 'top' just don't tend to pick them out at all, of course. But I do
> note that the LD [M] phase seems to be entirely serialized with the
> 'CC [M]' phase. Does it really have to be?
>
>                Linus

The last CC[M] and LD[M] stages are very fast, because
the auto-generated *.mod.c files are tiny.



CC [M]     foo.mod.o
CC [M]     bar.mod.o

can be compiled in parallel.


LD [M]     foo.o
LD [M]     bar.o

can be linked in parallel.


So, I believe we can benefit from parallel building.



-- 
Best Regards
Masahiro Yamada

Re: [GIT PULL] Kbuild fixes for v6.15-rc7

[Linus Torvalds]

Masahiro,

On Wed, 14 May 2025 at 22:15, Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> Please pull some Kbuild fixes.

So this isn't actually about your latest kbuild fixes, but more a
generic kbuild thing now that the merge window has opened..

My build times have gone up by a *lot* lately (and by "lately" I don't
mean very recently, but in the last month or so), to the point that
I'm now finding it to be a real annoyance due to it slowing down the
merge window for me.

I did a profile on the load just to check, and it's all just gcc
(well, cc1 mainly), so it's not that something else like ccache got
enabled.

(Things like ccache can speed up builds in other scenarios, but I've
also seen it slow things down a lot for the worst-case sitation when
header files change, so I wanted to verify that it's not just that)

So it's just that we're spending a lot more time in the compiler.

I haven't tried to bisect it, and now I'm too busy to, but the two
main suspects are either that KASAN is now enabled and that just makes
the builds a lot slower (if so, it's my own fault - introduced by me
in commit 6f110a5e4f99 "Disable SLUB_TINY for build testing").

Or it's just that I upgraded to F42 and gcc-15 itself is perhaps just
that much slower.

Anyway, I'm emailing you exactly because I'm busy with the merge
window and this is causing issues for me but I don't have time to
figure it out - and so I'm going "maybe Masahiro is aware of this
already"

I also did notice while timing things that the final module link time
seems to have limited threading

For the rest of the build, I have load averages in the 100+ range
(because I build with "-j128"), but then the last few minutes it
starts dropping a lot when it's in the

  CC [M] ...

stage, and when it hits that Makefile.modfinal stage and we get

  LD [M] ...

stage it seems to have serialized completely and just spends 100% of
time in one single 'make' instance.

But that may be an effect of the link being so fast that 'ps' and
'top' just don't tend to pick them out at all, of course. But I do
note that the LD [M] phase seems to be entirely serialized with the
'CC [M]' phase. Does it really have to be?

               Linus

Re: [GIT PULL] Kbuild fixes for v6.15-rc7

[pr-tracker-bot]

The pull request you sent on Thu, 15 May 2025 14:14:26 +0900:

> git://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git tags/kbuild-fixes-v6.15

has been merged into torvalds/linux.git:
https://git.kernel.org/torvalds/c/088d13246a4672bc03aec664675138e3f5bff68c

Thank you!

-- 
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/prtracker.html

[GIT PULL] Kbuild fixes for v6.15-rc7

[Masahiro Yamada]

Hello Linus,

Please pull some Kbuild fixes.

Thank you.





The following changes since commit 82f2b0b97b36ee3fcddf0f0780a9a0825d52fec3:

  Linux 6.15-rc6 (2025-05-11 14:54:11 -0700)

are available in the Git repository at:

  git://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git
tags/kbuild-fixes-v6.15

for you to fetch changes up to e0cd396d899805d56df91b989f8efad3a36df0da:

  kbuild: fix typos "module.builtin" to "modules.builtin" (2025-05-12
15:04:52 +0900)

----------------------------------------------------------------
Kbuild fixes for v6.15

 - Add proper pahole version dependency to CONFIG_GENDWARFKSYMS to avoid
   module loading errors

 - Fix UAPI header tests for the OpenRISC architecture

 - Add dependency on the libdw package in Debian and RPM packages

 - Disable -Wdefault-const-init-unsafe warnings on Clang

 - Make "make clean ARCH=um" also clean the arch/x86/ directory

 - Revert the use of -fmacro-prefix-map=, which causes issues with
   debugger usability

----------------------------------------------------------------
Masahiro Yamada (4):
      um: let 'make clean' properly clean underlying SUBARCH as well
      init: remove unused CONFIG_CC_CAN_LINK_STATIC
      kbuild: fix dependency on sorttable
      kbuild: fix typos "module.builtin" to "modules.builtin"

Nathan Chancellor (1):
      kbuild: Disable -Wdefault-const-init-unsafe

Randy Dunlap (1):
      usr/include: openrisc: don't HDRTEST bpf_perf_event.h

Sami Tolvanen (1):
      kbuild: Require pahole <v1.28 or >v1.29 with GENDWARFKSYMS on X86

Thomas Weißschuh (2):
      Revert "kbuild: make all file references relative to source root"
      Revert "kbuild, rust: use -fremap-path-prefix to make paths relative"

WangYuli (2):
      kbuild: deb-pkg: Add libdw-dev:native to Build-Depends-Arch
      kbuild: rpm-pkg: Add (elfutils-devel or libdw-devel) to BuildRequires

 Documentation/kbuild/reproducible-builds.rst | 17 +++++++++++++++++
 Makefile                                     |  3 +--
 arch/um/Makefile                             |  1 +
 init/Kconfig                                 |  5 -----
 kernel/module/Kconfig                        |  5 +++++
 scripts/Makefile.extrawarn                   | 12 ++++++++++++
 scripts/Makefile.vmlinux                     |  4 ++--
 scripts/Makefile.vmlinux_o                   |  4 ++--
 scripts/package/kernel.spec                  |  1 +
 scripts/package/mkdebian                     |  2 +-
 usr/include/Makefile                         |  4 ++++
 11 files changed, 46 insertions(+), 12 deletions(-)


-- 
Best Regards
Masahiro Yamada