linux-kernel - Re: [PATCH v2] kbuild: add configurable debug info level

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Date:   Sun, 14 Aug 2022 11:01:36 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Dmitrii Bundin <dmitrii.bundin.a@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Isabella Basso <isabbasso@...eup.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Fangrui Song <maskray@...gle.com>
Subject: Re: [PATCH v2] kbuild: add configurable debug info level

+CC: Fangrui Song <maskray@...gle.com>




On Sun, Aug 14, 2022 at 9:25 AM Dmitrii Bundin
<dmitrii.bundin.a@...il.com> wrote:
>
> Provides a way to configure debug info level (-glevel).
> Debug level 3 includes extra information such as macro definitions. With
> level 3 enabled it's possible to expand macros right from the debugging
> session in gdb simplifying debugging when complicated macros involved.
> The default level is set to 2 to not change the default build behavior.
>
> Signed-off-by: Dmitrii Bundin <dmitrii.bundin.a@...il.com>
> ---
>
> Changes in v2: https://lore.kernel.org/all/20220804223504.4739-1-dmitrii.bundin.a@gmail.com/
>   - Replace hardcoded -g3 with a configurable debug info level
>
>  lib/Kconfig.debug      | 11 +++++++++++
>  scripts/Makefile.debug |  2 +-
>  2 files changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 2e24db4bff19..a17c12c20290 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -304,6 +304,17 @@ config DEBUG_INFO_REDUCED
>           DEBUG_INFO build and compile times are reduced too.
>           Only works with newer gcc versions.
>
> +config DEBUG_INFO_LEVEL
> +       int "Debug info level"
> +       range 0 3
> +       default "2"
> +       help
> +         Sets the level of how much debug information to generate (-glevel).
> +         Level 1 produces minimal debug information without including information
> +         about local variables. Level 3 includes extra information like macro
> +         definitions. Setting up level 3 will require significantly more disk
> +         space and increase built time. Level 0 produces no debug information.
> +



We already have CONFIG_DEBUG_INFO_NONE to
disable the debug info.


The combination of CONFIG_DEBUG_INFO=y and
CONFIG_DEBUG_INFO_LEVEL=0  (-g0)
would emulate CONFIG_DEBUG_INFO_NONE ?



Using 'int' does not look sensible to me.





>  config DEBUG_INFO_COMPRESSED
>         bool "Compressed debugging information"
>         depends on $(cc-option,-gz=zlib)
> diff --git a/scripts/Makefile.debug b/scripts/Makefile.debug
> index 9f39b0130551..28beffc42e71 100644
> --- a/scripts/Makefile.debug
> +++ b/scripts/Makefile.debug
> @@ -3,7 +3,7 @@ DEBUG_CFLAGS    :=
>  ifdef CONFIG_DEBUG_INFO_SPLIT
>  DEBUG_CFLAGS   += -gsplit-dwarf
>  else
> -DEBUG_CFLAGS   += -g
> +DEBUG_CFLAGS   += -g$(CONFIG_DEBUG_INFO_LEVEL)
>  endif
>
>  ifndef CONFIG_AS_IS_LLVM
> --
> 2.17.1
>


I want to consult Fangrui Song for this part.


With this Makefile code, CONFIG_DEBUG_INFO_SPLIT
takes the presidency over CONFIG_DEBUG_INFO_LEVEL.


When CONFIG_DEBUG_INFO_SPLIT is enabled (-gsplit-dwarf),
it always uses the default -g2 level.
CONFIG_DEBUG_INFO_LEVEL is just ignored silently.



It might be sensible in older GCC/Clang behavior because
-gsplit-dwarf implied -g2.


But, with this commit:
https://reviews.llvm.org/D80391

-gsplit-dwarf and -g<level> are orthogonal
for GCC 11+/Clang 12+, correct?


I think "splitting debug files" and "debug level"
should be controlled independently.
(but it depends on the compiler version, if I understood correctly)






--
Best Regards
Masahiro Yamada