Re: [PATCH] Makefile: support compressed debug info

From: Sedat Dilek
Date: Mon May 04 2020 - 12:25:09 EST

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On Mon, May 4, 2020 at 5:13 AM Nick Desaulniers
<nick.desaulniers@xxxxxxxxx> 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
>

Hi Nick,

thanks for the patch.

I have slightly modified it to adapt to Linux v5.7-rc4 (what was your base?).

Which linker did you use and has it an impact if you switch from
ld.bfd to ld.lld?

I tried a first normal run and in a 2nd one with
CONFIG_DEBUG_INFO_COMPRESSED=y both with clang-10 and ld.lld-10.

My numbers (sizes in MiB):

[ diffconfig ]

$ scripts/diffconfig /boot/config-5.7.0-rc4-1-amd64-clang
/boot/config-5.7.0-rc4-2-amd64-clang
BUILD_SALT "5.7.0-rc4-1-amd64-clang" -> "5.7.0-rc4-2-amd64-clang"
+DEBUG_INFO_COMPRESSED y

[ compiler and linker ]

$ clang-10 -v
ClangBuiltLinux clang version 10.0.1
(https://github.com/llvm/llvm-project
92d5c1be9ee93850c0a8903f05f36a23ee835dc2)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /home/dileks/src/llvm-toolchain/install/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/10
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/9
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/10
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Candidate multilib: x32;@mx32
Selected multilib: .;@m64

$ ld.lld-10 -v
LLD 10.0.1 (https://github.com/llvm/llvm-project
92d5c1be9ee93850c0a8903f05f36a23ee835dc2) (compatible with GNU
linkers)

[ sizes vmlinux ]

$ du -m 5.7.0-rc4-*/vmlinux*
409 5.7.0-rc4-1-amd64-clang/vmlinux
7 5.7.0-rc4-1-amd64-clang/vmlinux.compressed
404 5.7.0-rc4-1-amd64-clang/vmlinux.o
324 5.7.0-rc4-2-amd64-clang/vmlinux
7 5.7.0-rc4-2-amd64-clang/vmlinux.compressed
299 5.7.0-rc4-2-amd64-clang/vmlinux.o

[ readelf (.debug_info as example) ]

$ readelf -S vmlinux.o
[33] .debug_info PROGBITS 0000000000000000 01d6a5e8
0000000006be1ee6 0000000000000000 0 0 1

$ readelf -S vmlinux.o
[33] .debug_info PROGBITS 0000000000000000 01749f18
0000000002ef04d2 0000000000000000 C 0 0 1 <---
XXX: "C (compressed)" Flag

Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
l (large), p (processor specific)

[ sizes linux-image debian packages ]

$ du -m 5.7.0-rc4-*/linux-image*.deb
47 5.7.0-rc4-1-amd64-clang/linux-image-5.7.0-rc4-1-amd64-clang_5.7.0~rc4-1~bullseye+dileks1_amd64.deb
424 5.7.0-rc4-1-amd64-clang/linux-image-5.7.0-rc4-1-amd64-clang-dbg_5.7.0~rc4-1~bullseye+dileks1_amd64.deb
47 5.7.0-rc4-2-amd64-clang/linux-image-5.7.0-rc4-2-amd64-clang_5.7.0~rc4-2~bullseye+dileks1_amd64.deb
771 5.7.0-rc4-2-amd64-clang/linux-image-5.7.0-rc4-2-amd64-clang-dbg_5.7.0~rc4-2~bullseye+dileks1_amd64.deb

[ sizes linux-git dir (compilation finished ]

5.7.0-rc4-1-amd64-clang: 17963 /home/dileks/src/linux-kernel/linux
5.7.0-rc4-2-amd64-clang: 14328 /home/dileks/src/linux-kernel/linux

[ xz compressed linux-image-dbg packages ]

$ file linux-image-5.7.0-rc4-1-amd64-clang-dbg_5.7.0~rc4-1~bullseye+dileks1_amd64.deb
linux-image-5.7.0-rc4-1-amd64-clang-dbg_5.7.0~rc4-1~bullseye+dileks1_amd64.deb:
Debian binary package (format 2.0), with control.tar.xz, data
compression xz
$ file linux-image-5.7.0-rc4-2-amd64-clang-dbg_5.7.0~rc4-2~bullseye+dileks1_amd64.deb
linux-image-5.7.0-rc4-2-amd64-clang-dbg_5.7.0~rc4-2~bullseye+dileks1_amd64.deb:
Debian binary package (format 2.0), with control.tar.xz, data
compression xz

[ file-lists ]

$ dpkg --contents
linux-image-5.7.0-rc4-1-amd64-clang-dbg_5.7.0~rc4-1~bullseye+dileks1_amd64.deb
| wc -l
4395
$ dpkg --contents
linux-image-5.7.0-rc4-2-amd64-clang-dbg_5.7.0~rc4-2~bullseye+dileks1_amd64.deb
| wc -l
4395

[ file-lists vmlinux ]

$ dpkg --contents
linux-image-5.7.0-rc4-1-amd64-clang-dbg_5.7.0~rc4-1~bullseye+dileks1_amd64.deb
| grep vmlinux
-rwxr-xr-x root/root 428588312 2020-05-04 06:15
./usr/lib/debug/lib/modules/5.7.0-rc4-1-amd64-clang/vmlinux
lrwxrwxrwx root/root 0 2020-05-04 06:15
./usr/lib/debug/boot/vmlinux-5.7.0-rc4-1-amd64-clang ->
../lib/modules/5.7.0-rc4-1-amd64-clang/vmlinux
lrwxrwxrwx root/root 0 2020-05-04 06:15
./usr/lib/debug/vmlinux-5.7.0-rc4-1-amd64-clang ->
lib/modules/5.7.0-rc4-1-amd64-clang/vmlinux

$ dpkg --contents
linux-image-5.7.0-rc4-2-amd64-clang-dbg_5.7.0~rc4-2~bullseye+dileks1_amd64.deb
| grep vmlinux
-rwxr-xr-x root/root 339341456 2020-05-04 12:24
./usr/lib/debug/lib/modules/5.7.0-rc4-2-amd64-clang/vmlinux
lrwxrwxrwx root/root 0 2020-05-04 12:24
./usr/lib/debug/boot/vmlinux-5.7.0-rc4-2-amd64-clang ->
../lib/modules/5.7.0-rc4-2-amd64-clang/vmlinux
lrwxrwxrwx root/root 0 2020-05-04 12:24
./usr/lib/debug/vmlinux-5.7.0-rc4-2-amd64-clang ->
lib/modules/5.7.0-rc4-2-amd64-clang/vmlinux

[ conclusion ]

As you can see there is a size-reduction in case of vmlinux/vmlinux.o
(debug) files...
...and my linux-git directory in total is smaller: 17963M vs. 14328M.

But the resulting linux-image-dbg file is much fatter: 424M vs. 711M.
XZ-compressing the gz/zlib-compressed vmlinux (debug) file results in
a fatter linux-image-dbg package.

For a usage of vmlinux (debug) in a VM scenario this might be nice but
seen from a debian repository perspective not.

For the sake of completeness: I have just installed and booted the
"normal" linux-image debian package - not the debug packages.

Thanks.

Regards,
- Sedat -

> Suggested-by: David Blaikie <blakie@xxxxxxxxxx>
> Signed-off-by: Nick Desaulniers <nick.desaulniers@xxxxxxxxx>
> ---
> Makefile | 5 +++++
> lib/Kconfig.debug | 9 +++++++++
> 2 files changed, 14 insertions(+)
>
> diff --git a/Makefile b/Makefile
> index 981eb902384b..313a054e5dc6 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -825,6 +825,11 @@ ifdef CONFIG_DEBUG_INFO_REDUCED
> DEBUG_CFLAGS += $(call cc-option, -femit-struct-debug-baseonly) \
> $(call cc-option,-fno-var-tracking)
> endif
> +
> +ifdef CONFIG_DEBUG_INFO_COMPRESSED
> +DEBUG_CFLAGS += -gz=zlib
> +KBUILD_LDFLAGS += --compress-debug-sections=zlib
> +endif
> endif
>
> KBUILD_CFLAGS += $(DEBUG_CFLAGS)
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index f6f9a039f736..1f4a47ba6c1b 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -213,6 +213,15 @@ 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 $(ld-option,--compress-debug-sections=zlib)
> + help
> + Compress the debug information using zlib. Requires GCC 5.0+ or Clang
> + 5.0+.
> +
> config DEBUG_INFO_SPLIT
> bool "Produce split debuginfo in .dwo files"
> depends on DEBUG_INFO
> --
> 2.17.1
>
> --
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Next message: Christian Brauner: "Re: [PATCH v3 1/3] nsproxy: add struct nsset"
Previous message: Johan Hovold: "Re: [PATCH] USB: serial: qcserial: Add DW5816e support"
In reply to: Nick Desaulniers: "[PATCH] Makefile: support compressed debug info"
Next in thread: Fangrui Song: "Re: [PATCH] Makefile: support compressed debug info"
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