[PATCH v4 1/3] block: Introduce flag BLK_MQ_F_DISABLE_FAIR_TAG_SHARING

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

The fair sharing algorithm has a negative performance impact for storage
devices for which the full queue depth is required to reach peak
performance, e.g. UFS devices. This is because it takes long after a
request queue became inactive until tags are reassigned to the active
request queue(s). Since making tag sharing fair is not needed if the
request processing latency is similar for all request queues, introduce
the hctx / tag set flag BLK_MQ_F_DISABLE_FAIR_TAG_SHARING.

Cc: Cc: Christoph Hellwig <hch@lst.de>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Keith Busch <kbusch@kernel.org>
Cc: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Cc: Yu Kuai <yukuai1@huaweicloud.com>
Cc: Ed Tsai <ed.tsai@mediatek.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 block/blk-mq-debugfs.c | 1 +
 block/blk-mq.h         | 3 ++-
 include/linux/blk-mq.h | 1 +
 3 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index 5cbeb9344f2f..f41408103106 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -198,6 +198,7 @@ static const char *const hctx_flag_name[] = {
 	HCTX_FLAG_NAME(NO_SCHED),
 	HCTX_FLAG_NAME(STACKING),
 	HCTX_FLAG_NAME(TAG_HCTX_SHARED),
+	HCTX_FLAG_NAME(DISABLE_FAIR_TAG_SHARING),
 };
 #undef HCTX_FLAG_NAME
 
diff --git a/block/blk-mq.h b/block/blk-mq.h
index f75a9ecfebde..eda6bd0611ea 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -416,7 +416,8 @@ static inline bool hctx_may_queue(struct blk_mq_hw_ctx *hctx,
 {
 	unsigned int depth, users;
 
-	if (!hctx || !(hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED))
+	if (!hctx || !(hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED) ||
+	    (hctx->flags & BLK_MQ_F_DISABLE_FAIR_TAG_SHARING))
 		return true;
 
 	/*
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 1ab3081c82ed..b12a0be839aa 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -662,6 +662,7 @@ enum {
 	 * or shared hwqs instead of 'mq-deadline'.
 	 */
 	BLK_MQ_F_NO_SCHED_BY_DEFAULT	= 1 << 7,
+	BLK_MQ_F_DISABLE_FAIR_TAG_SHARING = 1 << 8,
 	BLK_MQ_F_ALLOC_POLICY_START_BIT = 8,
 	BLK_MQ_F_ALLOC_POLICY_BITS = 1,