[PATCH v2] sbitmap: don't loop for find_next_zero_bit() for !round_robin

[PATCH v2] sbitmap: don't loop for find_next_zero_bit() for !round_robin

[Jens Axboe]

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.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

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

Re: [PATCH v2] sbitmap: don't loop for find_next_zero_bit() for !round_robin

[Omar Sandoval]

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@fb.com>

> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> ---
> 
> 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
>