block: improve batched tag allocation

Add a blk_mq_get_tags() helper, which uses the new sbitmap API for
allocating a batch of tags all at once. This both simplifies the block
code for batched allocation, and it is also more efficient than just
doing repeated calls into __sbitmap_queue_get().

This reduces the sbitmap overhead in peak runs from ~3% to ~1% and
yields a performanc increase from 6.6M IOPS to 6.8M IOPS for a single
CPU core.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq-tag.c

@@ -86,6 +86,21 @@ static int __blk_mq_get_tag(struct blk_mq_alloc_data *data,
 		return __sbitmap_queue_get(bt);
 }
 
+unsigned long blk_mq_get_tags(struct blk_mq_alloc_data *data, int nr_tags,
+			      unsigned int *offset)
+{
+	struct blk_mq_tags *tags = blk_mq_tags_from_data(data);
+	struct sbitmap_queue *bt = &tags->bitmap_tags;
+	unsigned long ret;
+
+	if (data->shallow_depth ||data->flags & BLK_MQ_REQ_RESERVED ||
+	    data->hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED)
+		return 0;
+	ret = __sbitmap_queue_get_batch(bt, nr_tags, offset);
+	*offset += tags->nr_reserved_tags;
+	return ret;
+}
+
 unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 {
 	struct blk_mq_tags *tags = blk_mq_tags_from_data(data);

block/blk-mq-tag.h

@@ -38,6 +38,8 @@ extern int blk_mq_init_bitmaps(struct sbitmap_queue *bitmap_tags,
 			       int node, int alloc_policy);
 
 extern unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data);
+unsigned long blk_mq_get_tags(struct blk_mq_alloc_data *data, int nr_tags,
+			      unsigned int *offset);
 extern void blk_mq_put_tag(struct blk_mq_tags *tags, struct blk_mq_ctx *ctx,
 			   unsigned int tag);
 extern int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,

block/blk-mq.c

@@ -354,6 +354,38 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
 	return rq;
 }
 
+static inline struct request *
+__blk_mq_alloc_requests_batch(struct blk_mq_alloc_data *data,
+		u64 alloc_time_ns)
+{
+	unsigned int tag, tag_offset;
+	struct request *rq;
+	unsigned long tags;
+	int i, nr = 0;
+
+	tags = blk_mq_get_tags(data, data->nr_tags, &tag_offset);
+	if (unlikely(!tags))
+		return NULL;
+
+	for (i = 0; tags; i++) {
+		if (!(tags & (1UL << i)))
+			continue;
+		tag = tag_offset + i;
+		tags &= ~(1UL << i);
+		rq = blk_mq_rq_ctx_init(data, tag, alloc_time_ns);
+		rq->rq_next = *data->cached_rq;
+		*data->cached_rq = rq;
+	}
+	data->nr_tags -= nr;
+
+	if (!data->cached_rq)
+		return NULL;
+
+	rq = *data->cached_rq;
+	*data->cached_rq = rq->rq_next;
+	return rq;
+}
+
 static struct request *__blk_mq_alloc_requests(struct blk_mq_alloc_data *data)
 {
 	struct request_queue *q = data->q;
@@ -388,43 +420,36 @@ static struct request *__blk_mq_alloc_requests(struct blk_mq_alloc_data *data)
 	if (!e)
 		blk_mq_tag_busy(data->hctx);
 
+	/*
+	 * Try batched alloc if we want more than 1 tag.
+	 */
+	if (data->nr_tags > 1) {
+		rq = __blk_mq_alloc_requests_batch(data, alloc_time_ns);
+		if (rq)
+			return rq;
+		data->nr_tags = 1;
+	}
+
 	/*
 	 * Waiting allocations only fail because of an inactive hctx.  In that
 	 * case just retry the hctx assignment and tag allocation as CPU hotplug
 	 * should have migrated us to an online CPU by now.
 	 */
-	do {
-		tag = blk_mq_get_tag(data);
-		if (tag == BLK_MQ_NO_TAG) {
-			if (data->flags & BLK_MQ_REQ_NOWAIT)
-				break;
-			/*
-			 * Give up the CPU and sleep for a random short time to
-			 * ensure that thread using a realtime scheduling class
-			 * are migrated off the CPU, and thus off the hctx that
-			 * is going away.
-			 */
-			msleep(3);
-			goto retry;
-		}
-
-		rq = blk_mq_rq_ctx_init(data, tag, alloc_time_ns);
-		if (!--data->nr_tags || e ||
-		    (data->hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED))
-			return rq;
-
-		/* link into the cached list */
-		rq->rq_next = *data->cached_rq;
-		*data->cached_rq = rq;
-		data->flags |= BLK_MQ_REQ_NOWAIT;
-	} while (1);
-
-	if (!data->cached_rq)
-		return NULL;
+	tag = blk_mq_get_tag(data);
+	if (tag == BLK_MQ_NO_TAG) {
+		if (data->flags & BLK_MQ_REQ_NOWAIT)
+			return NULL;
+		/*
+		 * Give up the CPU and sleep for a random short time to
+		 * ensure that thread using a realtime scheduling class
+		 * are migrated off the CPU, and thus off the hctx that
+		 * is going away.
+		 */
+		msleep(3);
+		goto retry;
+	}
 
-	rq = *data->cached_rq;
-	*data->cached_rq = rq->rq_next;
-	return rq;
+	return blk_mq_rq_ctx_init(data, tag, alloc_time_ns);
 }
 
 struct request *blk_mq_alloc_request(struct request_queue *q, unsigned int op,