linux-kernel - Re: [PATCH 2/2] x86, Makefile: Add new generic x86-64 settings v2/v3/v4

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Message-ID: <YR1KDD/azm1IDr9u@zn.tnic>
Date:   Wed, 18 Aug 2021 19:59:00 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     torvic9@...lbox.org
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "clang-built-linux@...glegroups.com" 
        <clang-built-linux@...glegroups.com>,
        "graysky@...hlinux.us" <graysky@...hlinux.us>,
        "masahiroy@...nel.org" <masahiroy@...nel.org>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH 2/2] x86, Makefile: Add new generic x86-64 settings
 v2/v3/v4

On Wed, Aug 18, 2021 at 05:08:40PM +0200, torvic9@...lbox.org wrote:
> I ran some quick checks and benchmarks, and your doubts seem to be
> justified. A 5.14-rc6 kernel compiled with the default 'generic' and
> one built with 'x86-64-v3' even have the exact same uncompressed file
> size. Benchmarks were inconclusive as well.

Lemme preface this with a IMHO:

Yeah, those -march machine-specific compiler switches don't really
show any perf improvements for kernels because, well, if you look at
the instruction stream a kernel executes, there's not really a whole
lot left to optimize after -O2.

Also, the percentage of time a machine spends in the kernel should be a
lot smaller (orders of magnitude) than in userspace - the operative word
being *should* here :-) - so there really isn't anything to optimize.

Not to say that there aren't a gazillion other places in the kernel that
could use more eyes and testing. ^Hint hint^

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette