linux-kernel - Re: [PATCH 4/9] select: Micro-optimise __estimate

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Message-ID: <20190919140547.GC2507@uranus.lan>
Date:   Thu, 19 Sep 2019 17:05:47 +0300
From:   Cyrill Gorcunov <gorcunov@...il.com>
To:     Dmitry Safonov <dima@...sta.com>
Cc:     linux-kernel@...r.kernel.org,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Adrian Reber <adrian@...as.de>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrei Vagin <avagin@...nvz.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        containers@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 4/9] select: Micro-optimise __estimate_accuracy()

On Mon, Sep 09, 2019 at 11:23:35AM +0100, Dmitry Safonov wrote:
> Shift on s64 is faster than division, use it instead.
> 
> As the result of the patch there is a hardly user-visible effect:
> poll(), select(), etc syscalls will be a bit more precise on ~2.3%
> than before because 1000 != 1024 :)
> 
> Signed-off-by: Dmitry Safonov <dima@...sta.com>
> ---
>  fs/select.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/select.c b/fs/select.c
> index 12cdefd3be2d..2477c202631e 100644
> --- a/fs/select.c
> +++ b/fs/select.c
> @@ -51,15 +51,14 @@
>  
>  static long __estimate_accuracy(ktime_t slack)
>  {
> -	int divfactor = 1000;
> -
>  	if (slack < 0)
>  		return 0;

Btw, don't you better use <= here?