Re: [PATCH] kconfig: prefer toolchain default for debug information choice

[Nathan Chancellor <nathan@kernel.org>]

Hi Thomas,

On Sun, Nov 24, 2024 at 04:58:04PM +0100, Thomas Weißschuh wrote:
> Kconfig by default chooses the first entry of a choice setting.
> For the "debug information" choice this is DEBUG_INFO_NONE which
> disables debug information completely.
> 
> The kconfig choice itself recommends to use "Toolchain default":
> 
> 	Choose which version of DWARF debug info to emit. If unsure,
> 	select "Toolchain default".
> 
> Align the actual configuration with the recommendation by providing an
> explicit default.
> 
> This also enables more codepaths from allmodconfig/allyesconfig which
> depend on debug information being available.
> 
> Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
> ---
>  lib/Kconfig.debug | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 5d9eca035d470f7ba0c5ff932c37fd5869174269..0aefcd103d9012cd8067e5594404358b0e977644 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -240,6 +240,7 @@ config AS_HAS_NON_CONST_ULEB128
>  choice
>  	prompt "Debug information"
>  	depends on DEBUG_KERNEL
> +	default DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
>  	help
>  	  Selecting something other than "None" results in a kernel image
>  	  that will include debugging info resulting in a larger kernel image.
> 
> ---
> base-commit: 9f16d5e6f220661f73b36a4be1b21575651d8833
> change-id: 20241124-kbuild-allconfig_debug_info-f7449ba15be6
> 
> Best regards,
> -- 
> Thomas Weißschuh <linux@weissschuh.net>
> 

I am not the biggest fan of this because it appears to have around a 5%
penalty in compilation times when I benchmarked building allmodconfig
with and without this change.

With LLVM 19.1.4:

  Benchmark 1: DEBUG_INFO_NONE
    Time (mean ± σ):     715.858 s ±  0.531 s    [User: 38038.311 s, System: 3718.784 s]
    Range (min … max):   715.271 s … 716.307 s    3 runs

  Benchmark 2: DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
    Time (mean ± σ):     760.749 s ±  0.172 s    [User: 40699.800 s, System: 3817.819 s]
    Range (min … max):   760.617 s … 760.943 s    3 runs

  Summary
    DEBUG_INFO_NONE ran
      1.06 ± 0.00 times faster than DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT

With GCC 14.2.0:

  Benchmark 1: DEBUG_INFO_NONE
    Time (mean ± σ):     830.524 s ±  0.342 s    [User: 43901.642 s, System: 4515.917 s]
    Range (min … max):   830.135 s … 830.777 s    3 runs

  Benchmark 2: DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
    Time (mean ± σ):     873.663 s ±  0.150 s    [User: 46102.416 s, System: 4968.065 s]
    Range (min … max):   873.565 s … 873.836 s    3 runs

  Summary
    DEBUG_INFO_NONE ran
      1.05 ± 0.00 times faster than DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT

I understand the desire to have CONFIG_DEBUG_INFO_BTF be selected with
allmodconfig for the sake of coverage but that is going to add up for
builders doing many builds a day.

Maybe we could add a fragment to kernel/configs for easily flipping
this? Another alternative that I have thought about recently is allowing
developers to specify a directory that holds out of tree config
fragments (KBUILD_FRAGMENTS_DIR?) that would be searched like
kernel/configs and arch/*/configs, so that people could maintain their
own fragments for easily doing something like:

  allmodconfig debug_info_btf.config

during configuration. Regardless though, if others find this new default
desirable, I am fine with it.

Cheers,
Nathan