Date:   Tue, 12 May 2020 10:59:13 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Nick Desaulniers <nick.desaulniers@...il.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Changbin Du <changbin.du@...el.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        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 ML <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] Makefile: support compressed debug info

On Tue, May 12, 2020 at 7:47 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> Hi Sedat,
>
>
> On Tue, May 5, 2020 at 1:25 AM Sedat Dilek <sedat.dilek@...il.com> wrote:
> >
> > On Mon, May 4, 2020 at 5:13 AM Nick Desaulniers
> > <nick.desaulniers@...il.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?
> >
> > 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.
>
>
> I also confirmed that, but this would not
> be a blocker of this patch.
>

Hi Masahiro,

No, it is not a blocker.
I have this patch now in my Linux v5.7-rc5 series.

I see a lot of more benefits concerning disc-usage - in my linux-git
and a reduced vmlinux file when I wanted to test with QEMU.

Feel free to add:
Tested-by: Sedat Dilek <sedat.dilek@...il.com>

- Sedat -

> Users can disable CONFIG_DEBUG_INFO_COMPRESSED
> if they care about the debug package size.
>
>
>
>
>
> --
> Best Regards
> Masahiro Yamada
>
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