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From: Masahiro Yamada <masahiroy@kernel.org>
Date: Mon, 22 Jan 2024 21:21:46 +0900
Message-ID: <CAK7LNAQQLP2jmy=mwvi9euFM=YOtKBDOJWQcO4Pr_J=Gtu5MFA@mail.gmail.com>
Subject: Re: [MAYBE REGRESSION] kbuild time of kernel compiling
To: Jingbo Xu <jefflexu@linux.alibaba.com>
Cc: linux-kbuild@vger.kernel.org, nathan@kernel.org, ndesaulniers@google.com, 
	nicolas@fjasle.eu, 
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Mon, Jan 22, 2024 at 5:32=E2=80=AFPM Jingbo Xu <jefflexu@linux.alibaba.c=
om> wrote:
>
> Hi,
>
> I noticed a regression of kbuild time in v6.7, especially when running
> `make` command when all images have already been compiled.  In v6.6 it
> takes ~3s, while in v6.7 it takes ~18s.
>
> I'm not sure if it's a known issue, or an extra action configurable with
> a "CONFIG_XX" option.
>
>
> Following is the kbuild time in v6.6 versus v7.7, with
> arch/x86/configs/x86_64_defconfig used here:
>
>
> v6.6
> ```
> make clean
> make olddefconfig
>
> # first full compiling
> $time make bzImage -j128 -s
>
> real    1m9.896s
> user    36m56.153s
> sys     4m21.748s
>
> # second time with image already compiled
> $time make bzImage -j128 -s
>
> real    0m2.776s
> user    0m13.823s
> sys     0m3.936s
> ```
>
>
> v6.7
> ```
> make clean
> make olddefconfig
>
> # first full compiling
> $time make bzImage -j128 -s
>
> real    1m22.865s
> user    37m25.977s
> sys     4m22.094s
>
> # second time with image already compiled
> $time make bzImage -j128 -s
>
> real    0m18.209s
> user    0m29.243s
> sys     0m4.330s
> ```
>
>
> I tried to bisect, while commit d4e175f2c460 ("Merge tag 'vfs-6.7.super'
> of gitolite.kernel.org:pub/scm/linux/kernel/git/vfs/vfs") is identified
> as the first bad commit, which is obviously wrong.  As I'm not familiar
> with the structure of the merge commit, the further investigation has
> not been made yet.


If you use olddefconfig, git-bisect will not produce the correct result.

'make olddefconfig' updates the existing .config.
The resulting .config depends on what you had
before running 'make olddefconfig'.

You need to use a deterministic target (e.g. defconfig)
during the git-bisect.


>
>
> Besides, it seems that it will take most of the time before "CALL
> scripts/checksyscalls.sh" is printed (with "INSTALL libsubcmd_headers"
> has already printed).
>
> $time make bzImage -j128
>   DESCEND objtool
>   INSTALL libsubcmd_headers
>
>   CALL    scripts/checksyscalls.sh
>   BUILD   arch/x86/boot/bzImage
> Kernel: arch/x86/boot/bzImage is ready  (#437)
>
> --
> Thanks,
> Jingbo
>


--
Best Regards
Masahiro Yamada