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Date: Thu, 21 May 2020 11:47:59 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
To: Nick Desaulniers <ndesaulniers@...gle.com>
Cc: Sedat Dilek <sedat.dilek@...il.com>,
Fangrui Song <maskray@...gle.com>,
Nick Clifton <nickc@...hat.com>,
David Blaikie <blakie@...gle.com>,
Michal Marek <michal.lkml@...kovi.net>,
Andrew Morton <akpm@...ux-foundation.org>,
Changbin Du <changbin.du@...el.com>,
Randy Dunlap <rdunlap@...radead.org>,
Stephen Rothwell <sfr@...b.auug.org.au>,
Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
Anshuman Khandual <anshuman.khandual@....com>,
Krzysztof Kozlowski <krzk@...nel.org>,
Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v2] Makefile: support compressed debug info
On Thu, May 21, 2020 at 4:36 AM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@...glegroups.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@...gle.com>
> Suggested-by: Fangrui Song <maskray@...gle.com>
Suggested-by -> Reviewed-by
https://patchwork.kernel.org/patch/11524939/#23349551
> Suggested-by: Nick Clifton <nickc@...hat.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@...il.com>
I do not see the source of this tag, either...
> Tested-by: Sedat Dilek <sedat.dilek@...il.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.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
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