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SubjectRe: [PATCH v3] Makefile: support compressed debug info
On Fri, May 22, 2020 at 12:00 AM 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 <blaikie@google.com>
> Suggested-by: Nick Clifton <nickc@redhat.com>
> Suggested-by: Sedat Dilek <sedat.dilek@gmail.com>
> Reviewed-by: Fangrui Song <maskray@google.com>
> Tested-by: Sedat Dilek <sedat.dilek@gmail.com>

Re-tested V3 on top of Linux v5.7-rc6+ with Clang/LLD v10.0.1-rc1.

- Sedat -

> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> 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 ++++++
> lib/Kconfig.debug | 17 +++++++++++++++++
> 2 files changed, 23 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..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
>

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