linux-kernel - Re: [PATCH] Makefile: support compressed debug info

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Message-ID: <CA+icZUUzLORp_TvDZiLO4WOdwEHTZF9vpi9BTHwQm-S3KDtMBw@mail.gmail.com>
Date:   Wed, 13 May 2020 18:33:30 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Fangrui Song <maskray@...gle.com>,
        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 Wed, May 13, 2020 at 4:52 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> Nick,
>
> On Wed, May 13, 2020 at 4:23 AM Nick Desaulniers
> <ndesaulniers@...gle.com> wrote:
> >
> > On Mon, May 11, 2020 at 10:54 PM Masahiro Yamada <masahiroy@...nel.org> 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.
> > > > >>
> > > Nick,
> > >
> > > I am OK with this patch.
> > >
> > > Fangrui provided the minimal requirement for
> > > --compress-debug-sections=zlib
> > >
> > >
> > > Is it worth recording in the help text?
> > > Do you want to send v2?
> >
> > Yes I'd like to record that information.  I can also record Sedat's
> > Tested-by tag.  Thank you for testing Sedat.
> >
> > I don't know what "linux-image-dbg file" are, or why they would be
> > bigger.  The size of the debug info is the primary concern with this
> > config.  It sounds like however that file is created might be
> > problematic.
>
>
>
> As Sedat explained, deb package data
> is compressed by xz, which is default.
>
> You can use another compression method,
> or disable compression if you desire.
>
>
>
> "man dpkg-deb" says as follows:
>
>  -Zcompress-type
>       Specify which compression type to use when building a package.
>       Allowed  values  are  gzip,  xz  (since  dpkg  1.15.6), and none
>       (default is xz).
>
>
>
> Kbuild supports KDEB_COMPRESS variable
> to change the compression method.
> See line 46 of scripts/package/builddeb.
>

Hi Masahiro,

thanks for the clarification and the informations.

With next RC I will add KDEB_COMPRESS=gzip to my build-script and report.

Regards,
- Sedat -

>
> If you are interested,
> try "make bindeb-pkg" with/without CONFIG_DEBUG_INFO_COMPRESSED,
> and compare the size of the generated debug package.
>
>
>
>
> As Sedat stated,
>
> (plain data) -> compress by gzip  ->  compress by xz
>
>    is often less efficient than
>
> (plain data) -> compress by xz
>
>
>
> I hope this is clearer.
>
>
>
>
>
>
>
>
> > Fangrui, I wasn't able to easily find what version of binutils first
> > added support.  Can you please teach me how to fish?
> >
> > Another question I had for Fangrui is, if the linker can compress
> > these sections, shouldn't we just have the linker do it, not the the
> > compiler and assembler?  IIUC the debug info can contain relocations,
> > so the linker would have to decompress these, perform relocations,
> > then recompress these?  I guess having the compiler and assembler
> > compress the debug info as well would minimize the size of the .o
> > files on disk.
> >
> > Otherwise I should add this flag to the assembler invocation, too, in
> > v2.  Thoughts?
> >
> > I have a patch series that enables dwarf5 support in the kernel that
> > I'm working up to.  I wanted to send this first.  Both roughly reduce
> > the debug info size by 20% each, though I haven't measured them
> > together, yet.  Requires ToT binutils because there have been many
> > fixes from reports of mine recently.
> > --
> > Thanks,
> > ~Nick Desaulniers
>
>
>
> --
> Best Regards
> Masahiro Yamada