On Wed, May 07, 2025 at 03:49:36PM -0600, Uday Shankar wrote:
> In update_alloc_hint_after_get, we wrap the new hint back to 0 one bit
> too early. This breaks round robin tag allocation (BLK_MQ_F_TAG_RR) -
> some tags get skipped, so we don't get round robin tags even in the
> simple case of single-threaded load on a single hctx. Fix the off-by-one
> in the wrapping condition so that round robin tag allocation works
> properly.
>
> The same pattern occurs in __sbitmap_get_word, so fix it there too.
>
> Signed-off-by: Uday Shankar <ushan...@purestorage.com>
> ---
> lib/sbitmap.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index
> d3412984170c03dc6600bbe53f130404b765ac5a..aa1cec78b9649f1f3e8ef2d617dd7ee724391a8c
> 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -51,7 +51,7 @@ static inline void update_alloc_hint_after_get(struct
> sbitmap *sb,
> } else if (nr == hint || unlikely(sb->round_robin)) {
> /* Only update the hint if we used it. */
> hint = nr + 1;
> - if (hint >= depth - 1)
> + if (hint >= depth)
> hint = 0;
> this_cpu_write(*sb->alloc_hint, hint);
This may help for round robin.
> }
> @@ -182,7 +182,7 @@ static int __sbitmap_get_word(unsigned long *word,
> unsigned long depth,
> break;
>
> hint = nr + 1;
> - if (hint >= depth - 1)
> + if (hint >= depth)
> hint = 0;
I guess round robin may need to return -1 if 'hint >= depth'.
Also the existing behavior starts from adding sbitmap, maybe Jens
has idea why hint is checked against 'depth -1' instead of 'depth'.
Thanks,
Ming
