linux-kernel - Re: x86/asm: __clear_user() micro-optimization (was: "Re: [GIT PULL] x86/asm changes for v4.18")

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Date:   Tue, 5 Jun 2018 08:47:24 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Alexey Dobriyan <adobriyan@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Anvin <hpa@...or.com>,
        Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: x86/asm: __clear_user() micro-optimization (was: "Re: [GIT PULL]
 x86/asm changes for v4.18")

On Tue, Jun 5, 2018 at 8:05 AM Ingo Molnar <mingo@...nel.org> wrote:
>
> Ok, fair point and agreed - if Alexey sends some measurements to back the change
> I'll keep this, otherwise queue up a revert.

I don't think it needs to be reverted, it's not like it's likely to
hurt on any modern CPU's. The issues I talked about are fairly
historical - barely even 64-bit cpus - and I'm not sure an extra uop
to carry a constant around even matters in that code sequence.

It was more a generic issue - any micro-optimization should be based
on numbers (and there should be some numbers in the commit message),
not on "this should be faster". Because while intuitively immediates
_should_ be faster than registers, that's simply not always "obviously
true". It _may_ be true. But numbers talk.

                  Linus