linux-kernel - Re: [PATCH] scatterlist: Speed up for_each

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Date:   Mon, 28 Oct 2019 11:17:34 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Sultan Alsawaf <sultan@...neltoast.com>
Cc:     "Martin K. Petersen" <martin.petersen@...cle.com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Ming Lei <ming.lei@...hat.com>,
        Gal Pressman <galpress@...zon.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] scatterlist: Speed up for_each_sg() loop macro

On Fri, Oct 25, 2019 at 02:33:58PM -0700, Sultan Alsawaf wrote:
> 
> Signed-off-by: Sultan Alsawaf <sultan@...neltoast.com>
>  include/linux/scatterlist.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
> index 556ec1ea2574..73f7fd6702d7 100644
> +++ b/include/linux/scatterlist.h
> @@ -146,7 +146,10 @@ static inline void sg_set_buf(struct scatterlist *sg, const void *buf,
>   * Loop over each sg element, following the pointer to a new list if necessary
>   */
>  #define for_each_sg(sglist, sg, nr, __i)	\
> -	for (__i = 0, sg = (sglist); __i < (nr); __i++, sg = sg_next(sg))
> +	for (__i = 0, sg = (sglist); __i < (nr);		\
> +	     likely(++__i % (SG_MAX_SINGLE_ALLOC - 1) ||	\
> +		    (__i + 1) >= (nr)) ? sg++ :			\
> +		    (sg = sg_chain_ptr(sg + 1)))

This is a big change in the algorithm, why are you sure it is OK?

Did you compare with just inlining sg_net?

Jason