linux-kernel - Re: [PATCH 0/2] x86: Remove ideal

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Date:   Tue, 30 Mar 2021 14:31:30 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Linus Torvalds <torvalds@...uxfoundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Anvin <hpa@...or.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-toolchains@...r.kernel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH 0/2] x86: Remove ideal_nops[]

On Sat, Mar 27, 2021 at 9:02 PM Linus Torvalds
<torvalds@...uxfoundation.org> wrote:
>
> On Sat, Mar 27, 2021 at 5:08 AM Sedat Dilek <sedat.dilek@...il.com> wrote:
> >
> > debian-5.10.19 as host-kernel:
> > 11655.755564957 seconds time elapsed
> >
> > dileks-5.12-rc3 plus x86-nops as host-kernel:
> > 11941.439350080 seconds time elapsed
>
> That's 2.5% - a huge difference. Particularly since kernel build times
> shouldn't even be that kernel-intensive.
>
> I think there's something else going on than the nops. Same config?
> There are likely many other differences between 5.10.19 and 5.12-rc3.
>
> So can you check just plain 5.12-rc3 and then 5.12-rc3 plus x86-nops,
> with otherwise identical configuration?
>

Hi Linus,

I re-checked my linux-config and custom patchset.

I had "kbuild: add CONFIG_VMLINUX_MAP expert option" in my queue and
build with CONFIG_VMLINUX_MAP=y.
This option generated here an approx. 30MiB big vmlinux.map file.
Cannot say how long this is taking in seconds but that can explain the
the time-diff.

[ The above option is helpful to analyze a recent Linux-kernel build
with CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y.
Always, I was able to build but not boot on bare metal with
CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y.
With a LLVM toolchain, of course. ]

( In the meantime Debian has a 5.20.26 kernel released - so if you
want I can re-test with Linux v5.12-rc5. )

Regards,
- Sedat -

[1] https://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git/commit/?h=kbuild&id=babd8cd96d333cb83c9b8abf4f01ab1f161d6ec4