Re: [PATCH] Makefile: support compressed debug info

From: Fangrui Song
Date: Mon May 04 2020 - 20:47:45 EST

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In reply to: Sedat Dilek: "Re: [PATCH] Makefile: support compressed debug info"
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On 2020-05-04, Sedat Dilek wrote:

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?

lld has supported the linker option --compress-debug-sections=zlib since
about 5.0.0 (https://reviews.llvm.org/D31941)

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+.
+

linker option --compress-debug-sections=zlib require binutils >= 2.26 (ld-option)
Assembler option --compress-debug-sections=zlib also require binutils >= 2.26 (cc-option,-gz=zlib)

config DEBUG_INFO_SPLIT
bool "Produce split debuginfo in .dwo files"
depends on DEBUG_INFO
--
2.17.1

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Next message: Daniel Jordan: "Re: [PATCH 5/7] mm: move zone iterator outside of deferred_init_maxorder()"
Previous message: Gavin Shan: "Re: [PATCH RFC 6/6] KVM: x86: Switch KVM guest to using interrupts for page ready APF delivery"
In reply to: Sedat Dilek: "Re: [PATCH] Makefile: support compressed debug info"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]