On Fri, Mar 8, 2019 at 1:25 PM Douglas Anderson <dianders@xxxxxxxxxxxx> wrote:
>
> As per the description of the old commit 3298b690b21c ("kbuild: Add a
> cache for generated variables"), calling the C compiler lots of times
> during the parsing stage of the Makefile can be a little slow. If you
> happen to have a C compiler whose fork/exec time isn't optimized
> (perhaps it was linked dynamically, or perhaps it's called through
> several wrappers, or ...) then this can be more than a little slow, it
> can be very slow.
>
> The above slowness was addressed with the old Makefile cache, but the
> cache was reverted in commit e08d6de4e532 ("kbuild: remove kbuild
> cache"). That commit indicated that we expected to get the speed
> gains back because "the result of compiler tests will be naturally
> cached in the .config file", but as of the latest mainline there are
> still lots of direct calls to the C compiler that happen in the
> Makefile parsing stage.
>
> Perhaps we could try re-introducing the Makefile cache and fix the
> problems it was causing, but this patch codes up another solution that
> gets some of the speed gains back, perhaps with less complexity.
>
> Specifically I observed that by timing the parsing stage of the
> Makefile by adding statements like this to the beginning and end of
> the Makefile:
> ZZZ := $(shell echo "$$(date '+%T.%N'): ..." >> /tmp/timing.txt)
>
> ...that three targets were spending 3 seconds each parsing the
> Makefile in my environment (building a Chrome OS kernel with clang)
> when it felt like they ought to be quick. These were: install,
> modules_install and kernelrelease
>
> I saw timings that looked like:
> 09:23:08.903204451: Parsing Makefile, goals: install
> 09:23:11.960515772: Done parsing Makefile, goals: install
>
> Given that the kbuild system doesn't sync the config for any of these
> options I believe it is safe to say that it can't compile any code.
> Thus if we can avoid the C compiler invocation here we'll save time.
>
> The simplest way to accomplish this is to neuter all the calls to the
> compiler by stubbing them out. After doing so (AKA applying this
> change) the same timing looks like:
> 09:28:04.929252271: Parsing Makefile, goals: install
> 09:28:05.107468449: Done parsing Makefile, goals: install
>
> During an incremental kernel build/install on Chrome OS we call
> install once, modules_install twice (once to keep unstripped), and
> kernelrelease once. Thus this saves ~12 seconds on an incremental
> kernel build/install. With my current setup, this brings it from ~40
> seconds to ~28 seconds. Re-introducing the Makefile cache would
> likely save us an extra ~3 seconds since we'd avoid the parsing in the
> compile stage.
>
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
>
> Makefile | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
>
> diff --git a/Makefile b/Makefile
> index f070e0d65186..5f7532d9be65 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -304,6 +304,20 @@ else
> scripts/Kbuild.include: ;
> include scripts/Kbuild.include
>
> +# If we can't sync the config then we know we can't compile code.
> +# Replace the slow option-testing commands with stubs to significantly
> +# speed up the Makefile parsing stage of the build.
> +ifeq ($(may-sync-config),0)
I think that breaks KBUILD_EXTMOD, which sets the variable to 0.
Guenter
> + ar-option :=
> + as-option :=
> + cc-disable-warning :=
> + cc-ifversion :=
> + cc-ldoption :=
> + cc-option :=
> + cc-option-yn := n
> + ld-option :=
> +endif
> +
> # Read KERNELRELEASE from include/config/kernel.release (if it exists)
> KERNELRELEASE = $(shell cat include/config/kernel.release 2> /dev/null)
> KERNELVERSION = $(VERSION)$(if $(PATCHLEVEL),.$(PATCHLEVEL)$(if $(SUBLEVEL),.$(SUBLEVEL)))$(EXTRAVERSION)
> --
> 2.21.0.360.g471c308f928-goog
>
