Re: [PATCH 4/9] select: Micro-optimise __estimate_accuracy()

[Cyrill Gorcunov]

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@xxxxxxxxxx>

> ---
>  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;
>  
> -	if (task_nice(current) > 0)
> -		divfactor = divfactor / 5;
> +	/* A bit more precise than 0.1% */
> +	slack = slack >> 10;
>  
> -	slack = ktime_divns(slack, divfactor);
> +	if (task_nice(current) > 0)
> +		slack = slack * 5;
>  
>  	if (slack > MAX_SLACK)
>  		return MAX_SLACK;

Compiler precompute constants so it doesn't do division here.
But I didn't read the series yet so I might be missing
something obvious.