Re: [PATCH v2] Makefile: support compressed debug info

[Masahiro Yamada <masahiroy@kernel.org>]

On Thu, May 21, 2020 at 4:36 AM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@googlegroups.com> wrote:
>
> As debug information gets larger and larger, it helps significantly save
> the size of vmlinux images to compress the information in the debug
> information sections. Note: this debug info is typically split off from
> the final compressed kernel image, which is why vmlinux is what's used
> in conjunction with GDB. Minimizing the debug info size should have no
> impact on boot times, or final compressed kernel image size.
>
> All of the debug sections will have a `C` flag set.
> $ readelf -S <object file>
>
> $ bloaty vmlinux.gcc75.compressed.dwarf4 -- \
>     vmlinux.gcc75.uncompressed.dwarf4
>
>     FILE SIZE        VM SIZE
>  --------------  --------------
>   +0.0%     +18  [ = ]       0    [Unmapped]
>  -73.3%  -114Ki  [ = ]       0    .debug_aranges
>  -76.2% -2.01Mi  [ = ]       0    .debug_frame
>  -73.6% -2.89Mi  [ = ]       0    .debug_str
>  -80.7% -4.66Mi  [ = ]       0    .debug_abbrev
>  -82.9% -4.88Mi  [ = ]       0    .debug_ranges
>  -70.5% -9.04Mi  [ = ]       0    .debug_line
>  -79.3% -10.9Mi  [ = ]       0    .debug_loc
>  -39.5% -88.6Mi  [ = ]       0    .debug_info
>  -18.2%  -123Mi  [ = ]       0    TOTAL
>
> $ bloaty vmlinux.clang11.compressed.dwarf4 -- \
>     vmlinux.clang11.uncompressed.dwarf4
>
>     FILE SIZE        VM SIZE
>  --------------  --------------
>   +0.0%     +23  [ = ]       0    [Unmapped]
>  -65.6%    -871  [ = ]       0    .debug_aranges
>  -77.4% -1.84Mi  [ = ]       0    .debug_frame
>  -82.9% -2.33Mi  [ = ]       0    .debug_abbrev
>  -73.1% -2.43Mi  [ = ]       0    .debug_str
>  -84.8% -3.07Mi  [ = ]       0    .debug_ranges
>  -65.9% -8.62Mi  [ = ]       0    .debug_line
>  -86.2% -40.0Mi  [ = ]       0    .debug_loc
>  -42.0% -64.1Mi  [ = ]       0    .debug_info
>  -22.1%  -122Mi  [ = ]       0    TOTAL
>
> For x86_64 defconfig + LLVM=1 (before):
> Elapsed (wall clock) time (h:mm:ss or m:ss): 3:22.03
> Maximum resident set size (kbytes): 43856
>
> For x86_64 defconfig + LLVM=1 (after):
> Elapsed (wall clock) time (h:mm:ss or m:ss): 3:32.52
> Maximum resident set size (kbytes): 1566776
>
> Suggested-by: David Blaikie <blakie@google.com>
> Suggested-by: Fangrui Song <maskray@google.com>


Suggested-by -> Reviewed-by

https://patchwork.kernel.org/patch/11524939/#23349551



> Suggested-by: Nick Clifton <nickc@redhat.com>


I do not know where this tag came from.

Nick Clifton taught us the version rule of binutils,but did not state
anything about this patch itself.

https://patchwork.kernel.org/patch/11524939/#23355175


> Suggested-by: Sedat Dilek <sedat.dilek@gmail.com>

I do not see the source of this tag, either...



> Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---

 snip

> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -225,6 +225,21 @@ config DEBUG_INFO_REDUCED
>           DEBUG_INFO build and compile times are reduced too.
>           Only works with newer gcc versions.
>
> +config DEBUG_INFO_COMPRESSED
> +       bool "Compressed debugging information"
> +       depends on DEBUG_INFO
> +       depends on $(cc-option,-gz=zlib)
> +       depends on $(as-option,-Wa,--compress-debug-sections=zlib)

This does not work. (always false)
You cannot enable this option.

The comma between -Wa and --compress-debug-sections=zlib
is eaten by Kconfig parser because commas are delimiters
of function parameters.


Please write like this.

    depends on $(as-option,-Wa$(comma)--compress-debug-sections=zlib)





> +       depends on $(ld-option,--compress-debug-sections=zlib)
> +       help
> +         Compress the debug information using zlib.  Requires GCC 5.0+ or Clang
> +         5.0+, binutils 2.26+, and zlib.
> +
> +         Users of dpkg-deb via scripts/package/builddeb may
> +         wish to set the $KDEB_COMPRESS env var to "none" to avoid recompressing
> +         the debug info again with a different compression scheme, which can
> +         result in larger binaries.

No. This is not correct.

CONFIG_DEBUG_INFO_COMPRESSED compresses the only debug info part.
The other parts still get by benefit from the default KDEB_COMPRESS=xz.


The numbers are here:


CONFIG_DEBUG_INFO_COMPRESSED=y
-rw-r--r-- 1 masahiro masahiro 209077584 May 21 11:19
linux-image-5.7.0-rc5+-dbg_5.7.0-rc5+-26_amd64.deb


CONFIG_DEBUG_INFO_COMPRESSED=y and KDEB_COMPRESS=none
-rw-r--r-- 1 masahiro masahiro 643051712 May 21 11:22
linux-image-5.7.0-rc5+-dbg_5.7.0-rc5+-27_amd64.deb


CONFIG_DEBUG_INFO_COMPRESSED=n
-rw-r--r-- 1 masahiro masahiro 112200308 May 21 11:40
linux-image-5.7.0-rc5+-dbg_5.7.0-rc5+-30_amd64.deb




For the deb package size perspective,
it is better to keep KDEB_COMPRESS as default.

The main motivation for changing KDEB_COMPRESS
is the build speed.  (see commit 1a7f0a34ea7d05)




CONFIG_DEBUG_INFO_COMPRESSED has a downside
for the debug deb package, but we need to accept it.









-- 
Best Regards
Masahiro Yamada