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In-Reply-To: <20200520193637.6015-1-ndesaulniers@google.com>
From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Wed, 20 May 2020 16:21:17 -0700
Message-ID: <CAKwvOd=LvSSrg6ynWwnmg4oYS+Ti_rcUrQNudwQLCnEgt6Uzyg@mail.gmail.com>
Subject: Re: [PATCH v2] Makefile: support compressed debug info
To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     Sedat Dilek <sedat.dilek@gmail.com>,
        Fangrui Song <maskray@google.com>,
        Nick Clifton <nickc@redhat.com>,
        Michal Marek <michal.lkml@markovi.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Changbin Du <changbin.du@intel.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        David Blaikie <blaikie@google.com>
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Sender: linux-kernel-owner@vger.kernel.org
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On Wed, May 20, 2020 at 12:36 PM Nick Desaulniers
<ndesaulniers@google.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@google.com>

Sorry, I have mistyped David's email, it should be:
Suggested-by: David Blaikie <blaikie@google.com>
(I missed the first `i` in the last name)

> Suggested-by: Fangrui Song <maskray@google.com>
> Suggested-by: Nick Clifton <nickc@redhat.com>
> Suggested-by: Sedat Dilek <sedat.dilek@gmail.com>
> Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> Changes V1 -> V2:
> * rebase on linux-next.
> * Add assembler flags as per Fangrui.
> * Add note about KDEB_COMPRESS+scripts/package/builddeb
>   as per Sedat and Masahiro.
> * Add note about bintutils version requirements as per Nick C.
> * Add note about measured increased build time and max RSS.
>
>  Makefile          |  6 ++++++
>  lib/Kconfig.debug | 15 +++++++++++++++
>  2 files changed, 21 insertions(+)
>
> diff --git a/Makefile b/Makefile
> index 71687bfe1cd9..be8835296754 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -822,6 +822,12 @@ 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_AFLAGS  += -Wa,--compress-debug-sections=zlib
> +KBUILD_LDFLAGS += --compress-debug-sections=zlib
> +endif
> +
>  KBUILD_CFLAGS += $(DEBUG_CFLAGS)
>  export DEBUG_CFLAGS
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index b8f023e054b9..5a423cbfaea4 100644
> --- 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)
> +       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.
> +
>  config DEBUG_INFO_SPLIT
>         bool "Produce split debuginfo in .dwo files"
>         depends on DEBUG_INFO
> --
> 2.26.2.761.g0e0b3e54be-goog
>


-- 
Thanks,
~Nick Desaulniers