linux-kernel - Re: [PATCH 1/2] x86/nmi: Optimize the check for being in the repeat

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Message-ID: <CALCETrUFE0X89bmsO8-tcOjPgPXTVCM1URUSD+HaEsGesfq6XA@mail.gmail.com>
Date:   Thu, 9 Mar 2017 19:50:57 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/2] x86/nmi: Optimize the check for being in the
 repeat_nmi code

On Thu, Mar 9, 2017 at 7:49 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
>
>
> On March 9, 2017 9:42:57 PM EST, Andy Lutomirski <luto@...capital.net> wrote:
>>On Thu, Mar 9, 2017 at 2:42 PM, Steven Rostedt <rostedt@...dmis.org>
>>wrote:
>>> From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
>>>
>>> Linus mentioned that doing two compares can be replaced by a single
>>> compare. That is, instead of:
>>>
>>>    movq $repeat_nmi, %rdx
>>>    cmpq 8(%rsp), %rdx
>>>    ja not_in_region
>>>    movq $end_repeat_nmi, %rdx
>>>    cmpq 8(%rsp), %rdx
>>>    ja in_region
>>>
>>> we can replace that with:
>>>
>>>    movq 8(%rsp), %rdx
>>>    subq $repeat_nmi, %rdx
>>>    cmpq $end_repeat_nmi-repeat_nmi, %rdx
>>>    jb in_region
>>
>>Seems reasonable to me.  Good luck ever noticing the speedup :)
>>
>
> It had nothing to do with speedup. Linus said that the current code makes the assembly programmer in him die a little. I want to cure that.
>

One might argue that the world would be a better place if the assembly
programmer in some people died a little.