block: pre-allocate requests if plug is started and is a batch

The caller typically has a good (or even exact) idea of how many requests
it needs to submit. We can make the request/tag allocation a lot more
efficient if we just allocate N requests/tags upfront when we queue the
first bio from the batch.

Provide a new plug start helper that allows the caller to specify how many
IOs are expected. This sets plug->nr_ios, and we can use that for smarter
request allocation. The plug provides a holding spot for requests, and
request allocation will check it before calling into the normal request
allocation path.

The blk_finish_plug() is called, check if there are unused requests and
free them. This should not happen in normal operations. The exception is
if we get merging, then we may be left with requests that need freeing
when done.

This raises the per-core performance on my setup from ~5.8M to ~6.1M
IOPS.

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

2 files changed, 18 insertions, 2 deletions

diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 75d75657df21..0e941f217578 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -90,7 +90,10 @@ struct request {
 	struct bio *bio;
 	struct bio *biotail;
 
-	struct list_head queuelist;
+	union {
+		struct list_head queuelist;
+		struct request *rq_next;
+	};
 
 	/*
 	 * The hash is used inside the scheduler, and killed once the
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 472b4ab007c6..17705c970d7e 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -722,10 +722,17 @@ extern void blk_set_queue_dying(struct request_queue *);
  */
 struct blk_plug {
 	struct list_head mq_list; /* blk-mq requests */
-	struct list_head cb_list; /* md requires an unplug callback */
+
+	/* if ios_left is > 1, we can batch tag/rq allocations */
+	struct request *cached_rq;
+	unsigned short nr_ios;
+
 	unsigned short rq_count;
+
 	bool multiple_queues;
 	bool nowait;
+
+	struct list_head cb_list; /* md requires an unplug callback */
 };
 
 struct blk_plug_cb;
@@ -738,6 +745,7 @@ struct blk_plug_cb {
 extern struct blk_plug_cb *blk_check_plugged(blk_plug_cb_fn unplug,
 					     void *data, int size);
 extern void blk_start_plug(struct blk_plug *);
+extern void blk_start_plug_nr_ios(struct blk_plug *, unsigned short);
 extern void blk_finish_plug(struct blk_plug *);
 extern void blk_flush_plug_list(struct blk_plug *, bool);
 
@@ -772,6 +780,11 @@ long nr_blockdev_pages(void);
 struct blk_plug {
 };
 
+static inline void blk_start_plug_nr_ios(struct blk_plug *plug,
+					 unsigned short nr_ios)
+{
+}
+
 static inline void blk_start_plug(struct blk_plug *plug)
 {
 }