Re: [PATCH] scatterlist: Speed up for_each_sg() loop macro

[Ming Lei <ming.lei@redhat.com>]

On Fri, Oct 25, 2019 at 02:33:58PM -0700, Sultan Alsawaf wrote:
> From: Sultan Alsawaf <sultan@kerneltoast.com>
> 
> Scatterlists are chained in predictable arrays of up to
> SG_MAX_SINGLE_ALLOC sg structs in length. Using this knowledge, speed up
> for_each_sg() by using constant operations to determine when to simply
> increment the sg pointer by one or get the next sg array in the chain.
> 
> Rudimentary measurements with a trivial loop body show that this yields
> roughly a 2x performance gain.
> 
> The following simple test module proves the correctness of the new loop
> definition by testing all the different edge cases of sg chains:
> #include <linux/module.h>
> #include <linux/scatterlist.h>
> #include <linux/slab.h>
> 
> static int __init test_for_each_sg(void)
> {
> 	static const gfp_t gfp_flags = GFP_KERNEL | __GFP_NOFAIL;
>         struct scatterlist *sg;
>         struct sg_table *table;
>         long old = 0, new = 0;
>         unsigned int i, nents;
> 
>         table = kmalloc(sizeof(*table), gfp_flags);
>         for (nents = 1; nents <= 3 * SG_MAX_SINGLE_ALLOC; nents++) {
>                 BUG_ON(sg_alloc_table(table, nents, gfp_flags));
>                 for (sg = table->sgl; sg; sg = sg_next(sg))
>                         old ^= (long)sg;
>                 for_each_sg(table->sgl, sg, nents, i)
>                         new ^= (long)sg;
>                 sg_free_table(table);
>         }
> 
>         BUG_ON(old != new);
>         kfree(table);
>         return 0;
> }
> module_init(test_for_each_sg);
> 
> Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.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
> --- a/include/linux/scatterlist.h
> +++ 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)))
>  

sg_alloc_table_chained() may put a small sglist as the first chunk, then
chained with big one, and your patch breaks such usage.


Thanks,
Ming