[PATCH 11/14] block/mq-deadline: Reserve 25% of scheduler tags for synchronous requests

[Bart Van Assche <bvanassche@acm.org>]

For interactive workloads it is important that synchronous requests are
not delayed. Hence reserve 25% of scheduler tags for synchronous requests.
This patch still allows asynchronous requests to fill the hardware queues
since blk_mq_init_sched() makes sure that the number of scheduler requests
is the double of the hardware queue depth. From blk_mq_init_sched():

	q->nr_requests = 2 * min_t(unsigned int, q->tag_set->queue_depth,
				   BLKDEV_MAX_RQ);

Cc: Damien Le Moal <damien.lemoal@wdc.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Cc: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 block/mq-deadline.c | 52 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 52 insertions(+)

diff --git a/block/mq-deadline.c b/block/mq-deadline.c
index 1d1bb7a41d2a..a7d0584437d1 100644
--- a/block/mq-deadline.c
+++ b/block/mq-deadline.c
@@ -67,6 +67,7 @@ struct deadline_data {
 	int fifo_batch;
 	int writes_starved;
 	int front_merges;
+	u32 async_depth;
 
 	spinlock_t lock;
 	spinlock_t zone_lock;
@@ -397,6 +398,44 @@ static struct request *dd_dispatch_request(struct blk_mq_hw_ctx *hctx)
 	return rq;
 }
 
+/*
+ * Called by __blk_mq_alloc_request(). The shallow_depth value set by this
+ * function is used by __blk_mq_get_tag().
+ */
+static void dd_limit_depth(unsigned int op, struct blk_mq_alloc_data *data)
+{
+	struct deadline_data *dd = data->q->elevator->elevator_data;
+
+	/* Do not throttle synchronous reads. */
+	if (op_is_sync(op) && !op_is_write(op))
+		return;
+
+	/*
+	 * Throttle asynchronous requests and writes such that these requests
+	 * do not block the allocation of synchronous requests.
+	 */
+	data->shallow_depth = dd->async_depth;
+}
+
+/* Called by blk_mq_update_nr_requests(). */
+static void dd_depth_updated(struct blk_mq_hw_ctx *hctx)
+{
+	struct request_queue *q = hctx->queue;
+	struct deadline_data *dd = q->elevator->elevator_data;
+	struct blk_mq_tags *tags = hctx->sched_tags;
+
+	dd->async_depth = max(1UL, 3 * q->nr_requests / 4);
+
+	sbitmap_queue_min_shallow_depth(tags->bitmap_tags, dd->async_depth);
+}
+
+/* Called by blk_mq_init_hctx() and blk_mq_init_sched(). */
+static int dd_init_hctx(struct blk_mq_hw_ctx *hctx, unsigned int hctx_idx)
+{
+	dd_depth_updated(hctx);
+	return 0;
+}
+
 static void dd_exit_sched(struct elevator_queue *e)
 {
 	struct deadline_data *dd = e->elevator_data;
@@ -733,6 +772,15 @@ static int deadline_starved_show(void *data, struct seq_file *m)
 	return 0;
 }
 
+static int dd_async_depth_show(void *data, struct seq_file *m)
+{
+	struct request_queue *q = data;
+	struct deadline_data *dd = q->elevator->elevator_data;
+
+	seq_printf(m, "%u\n", dd->async_depth);
+	return 0;
+}
+
 static void *deadline_dispatch_start(struct seq_file *m, loff_t *pos)
 	__acquires(&dd->lock)
 {
@@ -775,6 +823,7 @@ static const struct blk_mq_debugfs_attr deadline_queue_debugfs_attrs[] = {
 	DEADLINE_QUEUE_DDIR_ATTRS(write),
 	{"batching", 0400, deadline_batching_show},
 	{"starved", 0400, deadline_starved_show},
+	{"async_depth", 0400, dd_async_depth_show},
 	{"dispatch", 0400, .seq_ops = &deadline_dispatch_seq_ops},
 	{},
 };
@@ -783,6 +832,8 @@ static const struct blk_mq_debugfs_attr deadline_queue_debugfs_attrs[] = {
 
 static struct elevator_type mq_deadline = {
 	.ops = {
+		.depth_updated		= dd_depth_updated,
+		.limit_depth		= dd_limit_depth,
 		.insert_requests	= dd_insert_requests,
 		.dispatch_request	= dd_dispatch_request,
 		.prepare_request	= dd_prepare_request,
@@ -796,6 +847,7 @@ static struct elevator_type mq_deadline = {
 		.has_work		= dd_has_work,
 		.init_sched		= dd_init_sched,
 		.exit_sched		= dd_exit_sched,
+		.init_hctx		= dd_init_hctx,
 	},
 
 #ifdef CONFIG_BLK_DEBUG_FS