[PATCH] block: Remove unnecessary unlikely()

[PATCH] block: Remove unnecessary unlikely()

[Matthew Wilcox (Oracle)]

Jens added unlikely() thinking that this was an error path.  It's
actually just the end of the iteration, so does not warrant an
unlikely().

Fixes: 7bed6f3d08b7 ("block: Fix iterating over an empty bio with bio_for_each_folio_all")
Cc: stable@vger.kernel.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/bio.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/bio.h b/include/linux/bio.h
index 875d792bffff..1518f1201ddd 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -286,7 +286,7 @@ static inline void bio_first_folio(struct folio_iter *fi, struct bio *bio,
 {
 	struct bio_vec *bvec = bio_first_bvec_all(bio) + i;
 
-	if (unlikely(i >= bio->bi_vcnt)) {
+	if (i >= bio->bi_vcnt) {
 		fi->folio = NULL;
 		return;
 	}
-- 
2.43.0

Re: [PATCH] block: Remove unnecessary unlikely()

[Jens Axboe]

On 1/19/24 9:34 AM, Matthew Wilcox (Oracle) wrote:
> Jens added unlikely() thinking that this was an error path.  It's
> actually just the end of the iteration, so does not warrant an
> unlikely().

This is because the previous fix (or my attempt at least) didn't do the
i >= vcnt, it checked for an empty bio instead. Which then definitely
did make it an error/unlikely path, but obviously this one is not.

The bio iterator stuff has gotten terribly unwieldy and complicated, and
not very efficient either. But I guess that's a story for another
investigation...

-- 
Jens Axboe

Re: [PATCH] block: Remove unnecessary unlikely()

[Jens Axboe]

On 1/19/24 9:41 AM, Jens Axboe wrote:
> On 1/19/24 9:34 AM, Matthew Wilcox (Oracle) wrote:
>> Jens added unlikely() thinking that this was an error path.  It's
>> actually just the end of the iteration, so does not warrant an
>> unlikely().
> 
> This is because the previous fix (or my attempt at least) didn't do the
> i >= vcnt, it checked for an empty bio instead. Which then definitely
> did make it an error/unlikely path, but obviously this one is not.

Just out of curiosity, I did some branch profiling on just normal
operations of on my box. Of the ~900K times we hit this path,
10% of them ended up in that branch, and 90% of them did not.
While it's not an error path, that does seem rather unlikely. Sure, for
single entries, it'll be hit 50% of the time, but for most normal IO
it'd definitely be less than 50%, and as per above non-scientif
profiling, it's around 10%.

-- 
Jens Axboe