linux-kernel - Re: [PATCH] block: simplify blksize

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Date:   Wed, 26 Oct 2022 09:29:21 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Dawei Li <set_pte_at@...look.com>, axboe@...nel.dk
Cc:     hch@....de, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: simplify blksize_bits() implementation

On 10/26/22 08:14, Dawei Li wrote:
> Convert current looping-based implementation into bit operation,
> which can bring improvement for:
> 
> 1) bitops is more efficient for its arch-level optimization.

As far as I know blksize_bits() is not used in the hot path so 
performance of this function is not critical.

> 2) Given that blksize_bits() is inline, _if_ @size is compile-time
> constant, it's possible that order_base_2() _may_ make output
> compile-time evaluated, depending on code context and compiler behavior.
> 
> Signed-off-by: Dawei Li <set_pte_at@...look.com>
> ---
>   include/linux/blkdev.h | 7 +------
>   1 file changed, 1 insertion(+), 6 deletions(-)
> 
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 50e358a19d98..117061c8b9a1 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1349,12 +1349,7 @@ static inline int blk_rq_aligned(struct request_queue *q, unsigned long addr,
>   /* assumes size > 256 */
>   static inline unsigned int blksize_bits(unsigned int size)
>   {
> -	unsigned int bits = 8;
> -	do {
> -		bits++;
> -		size >>= 1;
> -	} while (size > 256);
> -	return bits;
> +	return size > 512 ? order_base_2(size) : 9;
>   }

How about optimizing this function even further by eliminating the 
ternary operator, e.g. as follows (untested)?

         return order_base_2(size >> SECTOR_SHIFT) + SECTOR_SHIFT;

Thanks,

Bart.