[PATCH 1/2] block: make the fair sharing of tag configurable

[Ed Tsai]

Add a new queue flag QUEUE_FLAG_FAIR_TAG_SHARING to make the fair tag
sharing configurable.

Signed-off-by: Ed Tsai <ed.tsai@xxxxxxxxxxxx>
---
 block/blk-mq-debugfs.c | 1 +
 block/blk-mq-tag.c     | 1 +
 block/blk-mq.c         | 3 ++-
 include/linux/blkdev.h | 6 +++++-
 4 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index d23a8554ec4a..f03b8bfe63be 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -103,6 +103,7 @@ static const char *const blk_queue_flag_name[] = {
 	QUEUE_FLAG_NAME(RQ_ALLOC_TIME),
 	QUEUE_FLAG_NAME(HCTX_ACTIVE),
 	QUEUE_FLAG_NAME(NOWAIT),
+	QUEUE_FLAG_NAME(FAIR_TAG_SHARING),
 };
 #undef QUEUE_FLAG_NAME
 
diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index d6af9d431dc6..b8b36823f5f5 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -97,6 +97,7 @@ static int __blk_mq_get_tag(struct blk_mq_alloc_data *data,
 			    struct sbitmap_queue *bt)
 {
 	if (!data->q->elevator && !(data->flags & BLK_MQ_REQ_RESERVED) &&
+			blk_queue_fair_tag_sharing(data->q) &&
 			!hctx_may_queue(data->hctx, bt))
 		return BLK_MQ_NO_TAG;
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index f6dad0886a2f..f903107759f7 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1746,7 +1746,8 @@ static bool __blk_mq_alloc_driver_tag(struct request *rq)
 		bt = &rq->mq_hctx->tags->breserved_tags;
 		tag_offset = 0;
 	} else {
-		if (!hctx_may_queue(rq->mq_hctx, bt))
+		if (blk_queue_fair_tag_sharing(rq->q) &&
+		    !hctx_may_queue(rq->mq_hctx, bt))
 			return false;
 	}
 
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index b441e633f4dd..7fcb2356860d 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -561,10 +561,12 @@ struct request_queue {
 #define QUEUE_FLAG_NOWAIT       29	/* device supports NOWAIT */
 #define QUEUE_FLAG_SQ_SCHED     30	/* single queue style io dispatch */
 #define QUEUE_FLAG_SKIP_TAGSET_QUIESCE	31 /* quiesce_tagset skip the queue*/
+#define QUEUE_FLAG_FAIR_TAG_SHARING	32 /* fair allocation of shared tags */
 
 #define QUEUE_FLAG_MQ_DEFAULT	((1UL << QUEUE_FLAG_IO_STAT) |		\
 				 (1UL << QUEUE_FLAG_SAME_COMP) |	\
-				 (1UL << QUEUE_FLAG_NOWAIT))
+				 (1UL << QUEUE_FLAG_NOWAIT) |		\
+				 (1UL << QUEUE_FLAG_FAIR_TAG_SHARING))
 
 void blk_queue_flag_set(unsigned int flag, struct request_queue *q);
 void blk_queue_flag_clear(unsigned int flag, struct request_queue *q);
@@ -602,6 +604,8 @@ bool blk_queue_flag_test_and_set(unsigned int flag, struct request_queue *q);
 #define blk_queue_sq_sched(q)	test_bit(QUEUE_FLAG_SQ_SCHED, &(q)->queue_flags)
 #define blk_queue_skip_tagset_quiesce(q) \
 	test_bit(QUEUE_FLAG_SKIP_TAGSET_QUIESCE, &(q)->queue_flags)
+#define blk_queue_fair_tag_sharing(q) \
+	test_bit(QUEUE_FLAG_FAIR_TAG_SHARING, &(q)->queue_flags)
 
 extern void blk_set_pm_only(struct request_queue *q);
 extern void blk_clear_pm_only(struct request_queue *q);
-- 
2.18.0