On Wed, Jan 31, 2024 at 7:49 PM John Garry [off-list ref] wrote:

When hacking it is a waste of time and compute energy that we need to
rebuild much kernel code just for changing the head git commit, like this:

quoted

touch include/generated/utsrelease.h
time make  -j3

mkdir -p /home/john/mnt_sda4/john/kernel-dev2/tools/objtool && make O=/home/john/mnt_sda4/john/kernel-dev2 subdir=tools/objtool --no-print-directory -C objtool
  INSTALL libsubcmd_headers
  CALL    scripts/checksyscalls.sh
  CC      init/version.o
  AR      init/built-in.a
  CC      kernel/sys.o
  CC      kernel/module/main.o
  AR      kernel/module/built-in.a
  CC      drivers/base/firmware_loader/main.o
  CC      kernel/trace/trace.o
  AR      drivers/base/firmware_loader/built-in.a
  AR      drivers/base/built-in.a
  CC      net/ethtool/ioctl.o
  AR      kernel/trace/built-in.a
  AR      kernel/built-in.a
  AR      net/ethtool/built-in.a
  AR      net/built-in.a
  AR      drivers/built-in.a
  AR      built-in.a
  ...

Files like drivers/base/firmware_loader/main.c needs to be recompiled as
it includes generated/utsrelease.h for UTS_RELEASE macro, and utsrelease.h
is regenerated when the head commit changes.

Introduce global char uts_release[] in init/version.c, which this
mentioned code can use instead of UTS_RELEASE, meaning that we don't need
to rebuild for changing the head commit - only init/version.c needs to be
rebuilt. Whether all the references to UTS_RELEASE in the codebase are
proper is a different matter.

For an x86_64 defconfig build for this series on my old laptop, here is
before and after rebuild time:

before:
real    0m53.591s
user    1m1.842s
sys     0m9.161s

after:
real    0m37.481s
user    0m46.461s
sys     0m7.199s

Sending as an RFC as I need to test more of the conversions and I would
like to also convert more UTS_RELEASE users to prove this is proper
approach.

John Garry (4):
  init: Add uts_release
  tracing: Use uts_release
  net: ethtool: Use uts_release
  firmware_loader: Use uts_release

 drivers/base/firmware_loader/main.c | 39 +++++++++++++++++++++++------
 include/linux/utsname.h             |  1 +
 init/version.c                      |  3 +++
 kernel/trace/trace.c                |  4 +--
 net/ethtool/ioctl.c                 |  4 +--
 5 files changed, 39 insertions(+), 12 deletions(-)

--
2.35.3

As you see, several drivers store UTS_RELEASE in their driver data,
and even print it in debug print.


I do not see why it is useful.
As you discussed in 3/4, if UTS_RELEASE is unneeded,
it is better to get rid of it.


If such version information is useful for drivers, the intention is
whether the version of the module, or the version of vmlinux.
That is a question.
They differ when CONFIG_MODVERSION.


When module developers intend to printk the git version
from which the module was compiled from,
presumably they want to use UTS_RELEASE, which
was expanded at the compile time of the module.

If you replace it with uts_release, it is the git version
of vmlinux.


Of course, the replacement is safe for always-builtin code.



Lastly, we can avoid using UTS_RELEASE without relying
on your patch.



For example, commit 3a3a11e6e5a2bc0595c7e36ae33c861c9e8c75b1
replaced  UTS_RELEASE with init_uts_ns.name.release


So, is your uts_release a shorthand of init_uts_ns.name.release?



I think what you can contribute are:

 - Explore the UTS_RELEASE users, and check if you can get rid of it.

 - Where UTS_RELEASE is useful, consider if it is possible
   to replace it with init_uts_ns.name.release



-- 
Best Regards
Masahiro Yamada