linux-kernel - Re: [PATCH] Kbuild: Update config_data.gz only if KCONFIG

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Date:   Sat, 3 Apr 2021 14:11:07 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Elliot Berman <eberman@...eaurora.org>
Cc:     Michal Marek <michal.lkml@...kovi.net>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Matthias Maennich <maennich@...gle.com>,
        Trilok Soni <tsoni@...eaurora.org>
Subject: Re: [PATCH] Kbuild: Update config_data.gz only if KCONFIG_CONFIG
 materially changed

On Fri, Apr 2, 2021 at 7:45 AM Elliot Berman <eberman@...eaurora.org> wrote:
>
> If you update the timestamp of KCONFIG_CONFIG without actually changing
> anything, config_data.gz is re-generated and causes vmlinux to re-link.
> When Link Time Optimization is enabled, unnecessary re-linking of
> vmlinux is highly desirable since it adds several minutes to build time.
>
> Avoid touching config_data.gz by using filechk to compare the existing
> config_data.gz and update only if it changed.
>
> The .config can be touched, for instance, by a build script which
> installs the default defconfig and then applies a defconfig fragment on
> top.
>
> For a simple example on my x86 machine, I modified x86 default defconfig to set
> CONFIG_IKCONFIG=y and run:
>   make -j50 defconfig tiny.config vmlinux
>   make -j50 defconfig tiny.config vmlinux
> With this patch, vmlinux is not re-built as a result of config_data.gz
> change.
>
> Signed-off-by: Elliot Berman <eberman@...eaurora.org>
> ---
>  kernel/Makefile      | 2 +-
>  scripts/Makefile.lib | 2 ++
>  2 files changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/Makefile b/kernel/Makefile
> index 320f1f3..bd4e558 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -140,7 +140,7 @@ $(obj)/configs.o: $(obj)/config_data.gz
>
>  targets += config_data.gz
>  $(obj)/config_data.gz: $(KCONFIG_CONFIG) FORCE
> -       $(call if_changed,gzip)
> +       $(call filechk,gzip)


I do not think this is the right approach
because gzip is executed every time, even
if the time stamp is not changed.







>
>  $(obj)/kheaders.o: $(obj)/kheaders_data.tar.xz
>
> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> index a4fbaf8..81d3ec1 100644
> --- a/scripts/Makefile.lib
> +++ b/scripts/Makefile.lib
> @@ -282,6 +282,8 @@ cmd_objcopy = $(OBJCOPY) $(OBJCOPYFLAGS) $(OBJCOPYFLAGS_$(@F)) $< $@
>  quiet_cmd_gzip = GZIP    $@
>        cmd_gzip = cat $(real-prereqs) | $(KGZIP) -n -f -9 > $@
>
> +filechk_gzip = cat $(real-prereqs) | $(KGZIP) -n -f -9
> +
>  # DTC
>  # ---------------------------------------------------------------------------
>  DTC ?= $(objtree)/scripts/dtc/dtc
> --
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> a Linux Foundation Collaborative Project
>


-- 
Best Regards
Masahiro Yamada