linux-kernel - Re: [PATCH 5.11] block: optimise for_each

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Date:   Tue, 24 Nov 2020 19:37:29 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5.11] block: optimise for_each_bvec() advance

On Tue, Nov 24, 2020 at 10:21:23AM +0000, Pavel Begunkov wrote:
> Because of how for_each_bvec() works it never advances across multiple
> entries at a time, so bvec_iter_advance() is an overkill. Add
> specialised bvec_iter_advance_single() that is faster. It also handles
> zero-len bvecs, so can kill bvec_iter_skip_zero_bvec().
> 
>    text    data     bss     dec     hex filename
> before:
>   23977     805       0   24782    60ce lib/iov_iter.o
> before, bvec_iter_advance() w/o WARN_ONCE()
>   22886     600       0   23486    5bbe ./lib/iov_iter.o
> after:
>   21862     600       0   22462    57be lib/iov_iter.o
> 
> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> ---
>  include/linux/bvec.h | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/bvec.h b/include/linux/bvec.h
> index 2efec10bf792..4a304dfafa18 100644
> --- a/include/linux/bvec.h
> +++ b/include/linux/bvec.h
> @@ -121,18 +121,24 @@ static inline bool bvec_iter_advance(const struct bio_vec *bv,
>  	return true;
>  }
>  
> -static inline void bvec_iter_skip_zero_bvec(struct bvec_iter *iter)
> +static inline void bvec_iter_advance_single(const struct bio_vec *bv,
> +				struct bvec_iter *iter, unsigned int bytes)
>  {
> -	iter->bi_bvec_done = 0;
> -	iter->bi_idx++;
> +	unsigned int done = iter->bi_bvec_done + bytes;
> +
> +	if (done == bv[iter->bi_idx].bv_len) {
> +		done = 0;
> +		iter->bi_idx++;
> +	}
> +	iter->bi_bvec_done = done;
> +	iter->bi_size -= bytes;
>  }
>  
>  #define for_each_bvec(bvl, bio_vec, iter, start)			\
>  	for (iter = (start);						\
>  	     (iter).bi_size &&						\
>  		((bvl = bvec_iter_bvec((bio_vec), (iter))), 1);	\
> -	     (bvl).bv_len ? (void)bvec_iter_advance((bio_vec), &(iter),	\
> -		     (bvl).bv_len) : bvec_iter_skip_zero_bvec(&(iter)))
> +	     bvec_iter_advance_single((bio_vec), &(iter), (bvl).bv_len))
>  
>  /* for iterating one bio from start to end */
>  #define BVEC_ITER_ALL_INIT (struct bvec_iter)				\
> -- 
> 2.24.0
> 

Looks fine,

Reviewed-by: Ming Lei <ming.lei@...hat.com>

Thanks,
Ming