On Thu, Nov 29, 2018 at 12:34:12PM -0700, Jens Axboe wrote:
> If we aren't forced to do round robin tag allocation, just use the
> allocation hint to find the index for the tag word, don't use it for the
> offset inside the word.
Maybe also add "We're already fetching that cache line, so we might as
well check the whole word."
> This avoids a potential extra round trip in the
> bit looping.
>
> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
>
> ---
>
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index fdd1b8aa8ac6..2987b2ac8ed7 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -118,10 +118,19 @@ int sbitmap_get(struct sbitmap *sb, unsigned int alloc_hint, bool round_robin)
>
> index = SB_NR_TO_INDEX(sb, alloc_hint);
>
> + /*
> + * Unless we're doing round robin tag allocation, just use the
> + * alloc_hint to find the right word index. No point in looping
> + * twice in find_next_zero_bit() for that case.
> + */
> + if (round_robin)
> + alloc_hint = SB_NR_TO_BIT(sb, alloc_hint);
> + else
> + alloc_hint = 0;
> +
> for (i = 0; i < sb->map_nr; i++) {
> nr = __sbitmap_get_word(&sb->map[index].word,
> - sb->map[index].depth,
> - SB_NR_TO_BIT(sb, alloc_hint),
> + sb->map[index].depth, alloc_hint,
> !round_robin);
> if (nr != -1) {
> nr += index << sb->shift;
> @@ -130,12 +139,8 @@ int sbitmap_get(struct sbitmap *sb, unsigned int alloc_hint, bool round_robin)
>
> /* Jump to next index. */
> index++;
> - alloc_hint = index << sb->shift;
> -
> - if (index >= sb->map_nr) {
> + if (index >= sb->map_nr)
> index = 0;
> - alloc_hint = 0;
> - }
We need to set alloc_hint = 0 here for the round_robin case.
> }
>
> return nr;
>
> --
> Jens Axboe
>
