linux-kernel - Re: [PATCH 3/4] kbuild: create object directories simpler and faster

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Date:   Fri, 10 Nov 2017 07:26:40 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        Michal Marek <michal.lkml@...kovi.net>,
        Sam Ravnborg <sam@...nborg.org>,
        Douglas Anderson <dianders@...omium.org>,
        Michal Marek <mmarek@...e.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 3/4] kbuild: create object directories simpler and faster


* Masahiro Yamada <yamada.masahiro@...ionext.com> wrote:

> For the out-of-tree build, scripts/Makefile.build creates output
> directories, but this operation is not efficient.
> 
> scripts/Makefile.lib calculates obj-dirs as follows:
> 
>   obj-dirs := $(dir $(multi-objs) $(obj-y))
> 
> Please notice $(sort ...) is not used here.  Usually the resulted
> obj-dirs is as many "./" as objects.
> 
> For those duplicated paths, the following command is invoked.
> 
>   _dummy := $(foreach d,$(obj-dirs), $(shell [ -d $(d) ] || mkdir -p $(d)))
> 
> Then, the costly shell command is run over and over again.
> 
> I see many points for optimization:
> 
> [1] Use $(sort ...) to cut down duplicated paths before passing them
>     to system call
> [2] Use single $(shell ...) instead of repeating it with $(foreach ...)
>     This will reduce forking.
> [3] We can calculate obj-dirs more simply.  Most of objects are already
>     accumulated in $(targets).  So, $(dir $(targets)) is fine and more
>     comprehensive.
> 
> I also removed bad code in arch/x86/entry/vdso/Makefile.  This is now
> really unnecessary.
> 
> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> ---
> 
>  arch/x86/entry/vdso/Makefile |  4 ----
>  scripts/Makefile.build       | 15 ++++++---------
>  scripts/Makefile.host        | 11 -----------
>  scripts/Makefile.lib         |  5 -----
>  4 files changed, 6 insertions(+), 29 deletions(-)

I love not just the speedup, but the diffstat as well ;-)

Acked-by: Ingo Molnar <mingo@...nel.org>

Thanks,

	Ingo