linux-kernel - Re: [PATCH] Makefile.debug: support for -gz=zstd

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Date:   Sun, 23 Oct 2022 17:45:30 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Fangrui Song <maskray@...gle.com>,
        Sedat Dilek <sedat.dilek@....com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Terrell <terrelln@...com>, Tom Rix <trix@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        David Gow <davidgow@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Miguel Ojeda <ojeda@...nel.org>,
        Isabella Basso <isabbasso@...eup.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] Makefile.debug: support for -gz=zstd

On Thu, Oct 20, 2022 at 8:43 PM Nathan Chancellor <nathan@...nel.org> wrote:
>
> On Thu, Oct 20, 2022 at 10:56:49AM -0700, Nick Desaulniers wrote:
> > Make DEBUG_INFO_COMPRESSED a choice; DEBUG_INFO_UNCOMPRESSED is the
> > default, DEBUG_INFO_COMPRESSED uses zlib, DEBUG_INFO_COMPRESSED_ZSTD
> > uses zstd.
> >
> > Some quick N=1 measurements with du, /usr/bin/time -v, and bloaty:
> >
> > clang-16, x86_64 defconfig plus
> > CONFIG_DEBUG_INFO=y CONFIG_DEBUG_INFO_UNCOMPRESSED=y:
> > Elapsed (wall clock) time (h:mm:ss or m:ss): 0:55.43
> > 488M vmlinux
> > 27.6%   136Mi   0.0%       0    .debug_info
> >  6.1%  30.2Mi   0.0%       0    .debug_str_offsets
> >  3.5%  17.2Mi   0.0%       0    .debug_line
> >  3.3%  16.3Mi   0.0%       0    .debug_loclists
> >  0.9%  4.62Mi   0.0%       0    .debug_str
> >
> > clang-16, x86_64 defconfig plus
> > CONFIG_DEBUG_INFO=y CONFIG_DEBUG_INFO_COMPRESSED=y (zlib):
> > Elapsed (wall clock) time (h:mm:ss or m:ss): 1:00.35
> > 385M vmlinux
> > 21.8%  85.4Mi   0.0%       0    .debug_info
> >  2.1%  8.26Mi   0.0%       0    .debug_str_offsets
> >  2.1%  8.24Mi   0.0%       0    .debug_loclists
> >  1.9%  7.48Mi   0.0%       0    .debug_line
> >  0.5%  1.94Mi   0.0%       0    .debug_str
> >
> > clang-16, x86_64 defconfig plus
> > CONFIG_DEBUG_INFO=y CONFIG_DEBUG_INFO_COMPRESSED_ZSTD=y (zstd):
> > Elapsed (wall clock) time (h:mm:ss or m:ss): 0:59.69
> > 373M vmlinux
> > 21.4%  81.4Mi   0.0%       0    .debug_info
> >  2.3%  8.85Mi   0.0%       0    .debug_loclists
> >  1.5%  5.71Mi   0.0%       0    .debug_line
> >  0.5%  1.95Mi   0.0%       0    .debug_str_offsets
> >  0.4%  1.62Mi   0.0%       0    .debug_str
> >
> > That's only a 3.11% overall binary size savings over zlib, but at no
> > performance regression.
> >
> > Link: https://maskray.me/blog/2022-09-09-zstd-compressed-debug-sections
> > Link: https://maskray.me/blog/2022-01-23-compressed-debug-sections
> > Suggested-by: Sedat Dilek (DHL Supply Chain) <sedat.dilek@....com>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
>
> Reviewed-by: Nathan Chancellor <nathan@...nel.org>
>
> One small comment below.
>
> > ---
> >  lib/Kconfig.debug      | 26 +++++++++++++++++++++++++-
> >  scripts/Makefile.debug |  4 ++++
> >  2 files changed, 29 insertions(+), 1 deletion(-)
> >
> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > index 3fc7abffc7aa..4085ac77dc12 100644
> > --- a/lib/Kconfig.debug
> > +++ b/lib/Kconfig.debug
> > @@ -312,8 +312,22 @@ config DEBUG_INFO_REDUCED
> >         DEBUG_INFO build and compile times are reduced too.
> >         Only works with newer gcc versions.
> >
> > +choice
> > +     prompt "Compressed Debug information"
> > +     depends on DEBUG_KERNEL
>
> I think you can drop this depends. The entire block is in an
> 'if DEBUG_INFO', which can only be true if CONFIG_DEBUG_KERNEL is set
> because of the dependencies of the "Debug information" prompt above this
> file, which is how CONFIG_DEBUG_INFO is selected.
>

Yes, dropping this is good.

> > +     help
> > +       Compress the resulting debug info. Results in smaller debug info sections,
> > +       but requires that consumers are able to decompress the results.
> > +
> > +       If unsure, choose DEBUG_INFO_UNCOMPRESSED.
> > +
> > +config DEBUG_INFO_UNCOMPRESSED
> > +     bool "Don't compress debug information"
> > +     help
> > +       Don't compress debug info sections.
> > +
> >  config DEBUG_INFO_COMPRESSED

Also, I prefer...

DEBUG_INFO_COMPRESSED_NONE
DEBUG_INFO_COMPRESSED_ZLIB
DEBUG_INFO_COMPRESSED_ZSTD

Following...

$ ld.lld-16 -v
Debian LLD 16.0.0 (compatible with GNU linkers)

$ ld.lld-16 --help | grep compress-debug-sections
 --compress-debug-sections=[none,zlib,zstd]

> > -     bool "Compressed debugging information"
> > +     bool "Compress debugging information with zlib"
> >       depends on $(cc-option,-gz=zlib)
> >       depends on $(ld-option,--compress-debug-sections=zlib)
> >       help
> > @@ -327,6 +341,16 @@ config DEBUG_INFO_COMPRESSED
> >         preferable to setting $KDEB_COMPRESS to "none" which would be even
> >         larger.
> >

Anyone, checked $KDEB_COMPRESS - does this need changes?

-Sedat-

> > +config DEBUG_INFO_COMPRESSED_ZSTD
> > +     bool "Compress debugging information with zstd"
> > +     depends on $(cc-option,-gz=zstd)
> > +     depends on $(ld-option,--compress-debug-sections=zstd)
> > +     help
> > +       Compress the debug information using zstd.  Requires GCC 13.0+ or Clang
> > +       16.0+, binutils 2.40+, and zstd.
> > +
> > +endchoice # "Compressed Debug information"
> > +
> >  config DEBUG_INFO_SPLIT
> >       bool "Produce split debuginfo in .dwo files"
> >       depends on $(cc-option,-gsplit-dwarf)
> > diff --git a/scripts/Makefile.debug b/scripts/Makefile.debug
> > index 332c486f705f..8ac3379d2255 100644
> > --- a/scripts/Makefile.debug
> > +++ b/scripts/Makefile.debug
> > @@ -31,6 +31,10 @@ ifdef CONFIG_DEBUG_INFO_COMPRESSED
> >  DEBUG_CFLAGS += -gz=zlib
> >  KBUILD_AFLAGS        += -gz=zlib
> >  KBUILD_LDFLAGS       += --compress-debug-sections=zlib
> > +else ifdef CONFIG_DEBUG_INFO_COMPRESSED_ZSTD
> > +DEBUG_CFLAGS += -gz=zstd
> > +KBUILD_AFLAGS        += -gz=zstd
> > +KBUILD_LDFLAGS       += --compress-debug-sections=zstd
> >  endif
> >
> >  KBUILD_CFLAGS        += $(DEBUG_CFLAGS)
> > --
> > 2.38.0.135.g90850a2211-goog
> >