linux-kernel - Re: [PATCH v2] fs: optimise kiocb_set_rw

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Date:   Fri, 31 Jan 2020 16:01:40 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Alexander Viro <viro@...iv.linux.org.uk>, willy@...radead.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] fs: optimise kiocb_set_rw_flags()

On 1/17/2020 4:32 AM, Pavel Begunkov wrote:
> kiocb_set_rw_flags() generates a poor code with several memory writes
> and a lot of jumps. Help compilers to optimise it.
> 
> Tested with gcc 9.2 on x64-86, and as a result, it its output now is a
> plain code without jumps accumulating in a register before a memory
> write.

Humble ping

> 
> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> ---
> 
> v2: check for 0 flags in advance (Matthew Wilcox)
> 
>  include/linux/fs.h | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 98e0349adb52..22b46fc8fdfa 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -3402,22 +3402,28 @@ static inline int iocb_flags(struct file *file)
>  
>  static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
>  {
> +	int kiocb_flags = 0;
> +
> +	if (!flags)
> +		return 0;
>  	if (unlikely(flags & ~RWF_SUPPORTED))
>  		return -EOPNOTSUPP;
>  
>  	if (flags & RWF_NOWAIT) {
>  		if (!(ki->ki_filp->f_mode & FMODE_NOWAIT))
>  			return -EOPNOTSUPP;
> -		ki->ki_flags |= IOCB_NOWAIT;
> +		kiocb_flags |= IOCB_NOWAIT;
>  	}
>  	if (flags & RWF_HIPRI)
> -		ki->ki_flags |= IOCB_HIPRI;
> +		kiocb_flags |= IOCB_HIPRI;
>  	if (flags & RWF_DSYNC)
> -		ki->ki_flags |= IOCB_DSYNC;
> +		kiocb_flags |= IOCB_DSYNC;
>  	if (flags & RWF_SYNC)
> -		ki->ki_flags |= (IOCB_DSYNC | IOCB_SYNC);
> +		kiocb_flags |= (IOCB_DSYNC | IOCB_SYNC);
>  	if (flags & RWF_APPEND)
> -		ki->ki_flags |= IOCB_APPEND;
> +		kiocb_flags |= IOCB_APPEND;
> +
> +	ki->ki_flags |= kiocb_flags;
>  	return 0;
>  }
>  
> 

-- 
Pavel Begunkov