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 <CA+icZUUOaqeKeh6n4BJq2k6XQWAfNghUj57j42ZX5qyd3iOmLw@mail.gmail.com> <20200505004738.ew2lcp27c2n4jqia@google.com>
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From:   Masahiro Yamada <masahiroy@kernel.org>
Date:   Tue, 12 May 2020 14:53:01 +0900
Message-ID: <CAK7LNAR7-VMEWBcJ_Wd+61ZDHEa0gD8FaSs63YPu7m_FgH8Htg@mail.gmail.com>
Subject: Re: [PATCH] Makefile: support compressed debug info
To:     Fangrui Song <maskray@google.com>
Cc:     Sedat Dilek <sedat.dilek@gmail.com>,
        Nick Desaulniers <nick.desaulniers@gmail.com>,
        Michal Marek <michal.lkml@markovi.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Changbin Du <changbin.du@intel.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Clang-Built-Linux ML <clang-built-linux@googlegroups.com>
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Sender: linux-kernel-owner@vger.kernel.org
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On Tue, May 5, 2020 at 9:47 AM Fangrui Song <maskray@google.com> wrote:
>
>
> On 2020-05-04, Sedat Dilek wrote:
> >On Mon, May 4, 2020 at 5:13 AM Nick Desaulniers
> ><nick.desaulniers@gmail.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
> >>
> >
> >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@google.com>
> >> Signed-off-by: Nick Desaulniers <nick.desaulniers@gmail.com>
> >> ---
> >>  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)


Nick,

I am OK with this patch.

Fangrui provided the minimal requirement for
--compress-debug-sections=zlib


Is it worth recording in the help text?
Do you want to send v2?


-- 
Best Regards
Masahiro Yamada