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

Ming Lei
2019-10-29T02:25:52Z

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