On Thu, Nov 29, 2018 at 12:47:49PM -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. This avoids a potential extra round trip in the
> bit looping, and since we're fetching this cacheline, we may as well
> check the whole word from the start.
Reviewed-by: Omar Sandoval <osandov@xxxxxx>
> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
>
> ---
>
> Since v1:
>
> - Reset alloc_hint if we loop for the round-robin case
> - Update commit message
>
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index fdd1b8aa8ac6..45cab6bbc1c7 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;
> @@ -129,13 +138,9 @@ 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) {
> + alloc_hint = 0;
> + if (++index >= sb->map_nr)
> index = 0;
> - alloc_hint = 0;
> - }
> }
>
> return nr;
> --
> Jens Axboe
>
