Re: [PATCH] Kbuild: Update config_data.gz only if KCONFIG_CONFIG materially changed

[Elliot Berman]

On 4/2/2021 10:11 PM, Masahiro Yamada wrote:
> On Fri, Apr 2, 2021 at 7:45 AM Elliot Berman <eberman@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> > ---
> >   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.

Since .config is relatively small, gzip was quickly producing the same 
binary in multiple runs on my host. I thought about following 
gen_ikheaders.sh approach of comparing the md5sum, but I felt it was 
more complex than using filechk and re-compressing every time. I'll send 
a v2 tomorrow which follows gen_ikheaders.sh approach.

> >   $(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