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>

1 files changed, 15 insertions, 0 deletions

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 72a2724a4eee..c43b97201161 100644
--- a/block/blk-mq-tag.c
+++ b/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);