Re: [PATCH] block: optimise bvec_iter_advance()

[Arvind Sankar]

On Fri, Nov 29, 2019 at 12:04:37AM +0300, Pavel Begunkov wrote:
> bvec_iter_advance() is quite popular, but compilers fail to do proper
> alias analysis and optimise it good enough. The assembly is checked
> for gcc 9.2, x86-64.
> 
> - remove @iter->bi_size from min(...), as it's always less than @bytes.
> Modify at the beginning and forget about it.
> 
> - the compiler isn't able to collapse memory dependencies and remove
> writes in the loop. Help it by explicitely using local vars.
> 
> Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
> ---
>  include/linux/bvec.h | 23 ++++++++++++++---------
>  1 file changed, 14 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/bvec.h b/include/linux/bvec.h
> index a032f01e928c..7b2f05faae14 100644
> --- a/include/linux/bvec.h
> +++ b/include/linux/bvec.h
> @@ -87,26 +87,31 @@ struct bvec_iter_all {
>  static inline bool bvec_iter_advance(const struct bio_vec *bv,
>  		struct bvec_iter *iter, unsigned bytes)
>  {
> +	unsigned int done = iter->bi_bvec_done;
> +	unsigned int idx = iter->bi_idx;
> +
>  	if (WARN_ONCE(bytes > iter->bi_size,
>  		     "Attempted to advance past end of bvec iter\n")) {
>  		iter->bi_size = 0;
>  		return false;
>  	}
>  
> +	iter->bi_size -= bytes;
> +
>  	while (bytes) {
> -		const struct bio_vec *cur = bv + iter->bi_idx;
> -		unsigned len = min3(bytes, iter->bi_size,
> -				    cur->bv_len - iter->bi_bvec_done);
> +		const struct bio_vec *cur = bv + idx;
> +		unsigned int len = min(bytes, cur->bv_len - done);
>  
>  		bytes -= len;
> -		iter->bi_size -= len;
> -		iter->bi_bvec_done += len;
> -
> -		if (iter->bi_bvec_done == cur->bv_len) {
> -			iter->bi_bvec_done = 0;
> -			iter->bi_idx++;
> +		done += len;
> +		if (done == cur->bv_len) {
> +			idx++;
> +			done = 0;
>  		}
>  	}
> +
> +	iter->bi_idx = idx;
> +	iter->bi_bvec_done = done;
>  	return true;
>  }
>  
> -- 
> 2.24.0
> 

The loop can be simplified a bit further, as done has to be 0 once we go
beyond the current bio_vec. See below for the simplified version.

I also check if bi_size became zero so we can skip the rest of the
calculations in that case. If we want to preserve the current behavior of
updating iter->bi_idx and iter->bi_bvec_done even if bi_size is going to
become zero, the loop condition should change to

		while (bytes && bytes >= cur->bv_len)

to ensure that we don't try to access past the end of the bio_vec array.

diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index a032f01e928c..380d188dfbd2 100644
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -87,25 +87,26 @@ struct bvec_iter_all {
 static inline bool bvec_iter_advance(const struct bio_vec *bv,
 		struct bvec_iter *iter, unsigned bytes)
 {
+	unsigned int idx = iter->bi_idx;
+	const struct bio_vec *cur = bv + idx;
+
 	if (WARN_ONCE(bytes > iter->bi_size,
 		     "Attempted to advance past end of bvec iter\n")) {
 		iter->bi_size = 0;
 		return false;
 	}
 
-	while (bytes) {
-		const struct bio_vec *cur = bv + iter->bi_idx;
-		unsigned len = min3(bytes, iter->bi_size,
-				    cur->bv_len - iter->bi_bvec_done);
-
-		bytes -= len;
-		iter->bi_size -= len;
-		iter->bi_bvec_done += len;
-
-		if (iter->bi_bvec_done == cur->bv_len) {
-			iter->bi_bvec_done = 0;
-			iter->bi_idx++;
+	iter->bi_size -= bytes;
+	if (iter->bi_size != 0) {
+		bytes += iter->bi_bvec_done;
+		while (bytes >= cur->bv_len) {
+			bytes -= cur->bv_len;
+			idx++;
+			cur++;
 		}
+
+		iter->bi_idx = idx;
+		iter->bi_bvec_done = bytes;
 	}
 	return true;
 }