Re: [PATCH v4] Makefile: support compressed debug info

[Nick Desaulniers]

On Tue, May 26, 2020 at 10:03 AM Nick Desaulniers
<ndesaulniers@xxxxxxxxxx> 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
>
> Thanks to:
> Nick Clifton helped us to provide the minimal binutils version.
> Sedat Dilet found an increase in size of debug .deb package.
>
> Cc: Nick Clifton <nickc@xxxxxxxxxx>
> Cc: Sedat Dilek <sedat.dilek@xxxxxxxxx>
> Suggested-by: David Blaikie <blaikie@xxxxxxxxxx>
> Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>
> Tested-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> ---
> Changes V3 -> V4:
> * Add thanks line to commit message as per Masahiro.
> * Swap Sugguested-by to Cc for two lines in commit message, as per
>   Masahiro.
>
> Changes V2 -> V3:
> * Fix blaikie@'s email addr.
> * Fix Fangrui's Reviewed-by tag as per Masahiro.
> * Fix help text as per Masahiro.
> * Fix -Wa$(comma)foo as per Masahiro.
>
> 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 ++++++
>  arch/arm64/kernel/vdso32/Makefile |  2 +-

Sorry, I was wondering why Will and TGLX got cc'ed. My tree was dirty
when I amended ... was carrying another patch to send, please
disregard v4, and sorry for the noise.

>  lib/Kconfig.debug                 | 17 +++++++++++++++++
>  3 files changed, 24 insertions(+), 1 deletion(-)
>
> 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 += $(DEBUG_CFLAGS)
>  export DEBUG_CFLAGS
>
> diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso32/Makefile
> index 3964738ebbde..5fd7792d03fc 100644
> --- a/arch/arm64/kernel/vdso32/Makefile
> +++ b/arch/arm64/kernel/vdso32/Makefile
> @@ -135,7 +135,7 @@ c-obj-vdso-gettimeofday := vgettimeofday.o
>  asm-obj-vdso := sigreturn.o
>
>  ifneq ($(c-gettimeofday-y),)
> -VDSO_CFLAGS_gettimeofday_o += -include $(c-gettimeofday-y)
> +VDSO_CFLAGS_gettimeofday_o += -include $(c-gettimeofday-y) -marm
>  endif
>
>  VDSO_CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_FTRACE) -Os
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index b8f023e054b9..7fc82dcf814b 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -225,6 +225,23 @@ 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 $(as-option,-Wa$(comma)--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 find an increase in
> +         size of their debug .deb packages with this config set, due to the
> +         debug info being compressed with zlib, then the object files being
> +         recompressed with a different compression scheme. But this is still
> +         preferable to setting $KDEB_COMPRESS to "none" which would be even
> +         larger.
> +
>  config DEBUG_INFO_SPLIT
>         bool "Produce split debuginfo in .dwo files"
>         depends on DEBUG_INFO
> --
> 2.27.0.rc0.183.gde8f92d652-goog
>


-- 
Thanks,
~Nick Desaulniers