[RFC PATCH V2 04/17] blk-mq: don't reserve tags for admin queue

[Ming Lei <ming.lei@redhat.com>]

Not necessary to reserve tags for admin queue since there isn't
many inflight commands in admin queue usually.

This change won't starve admin queue too because each blocked queue
has equal priority to get one new tag when one driver tag is released,
no matter it is freed from any queue.

So that IO performance won't be affected after admin queue(shared tags
with IO queues) is introduced in the following patches.

Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Jianchao Wang <jianchao.w.wang@oracle.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: "James E.J. Bottomley" <jejb@linux.vnet.ibm.com>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: linux-scsi@vger.kernel.org
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq-tag.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 816923bf874d..7cd09fd16f5a 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -30,7 +30,8 @@ bool blk_mq_has_free_tags(struct blk_mq_tags *tags)
 bool __blk_mq_tag_busy(struct blk_mq_hw_ctx *hctx)
 {
 	if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state) &&
-	    !test_and_set_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
+	    !test_and_set_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state) &&
+	    !blk_queue_admin(hctx->queue))
 		atomic_inc(&hctx->tags->active_queues);
 
 	return true;
@@ -57,7 +58,8 @@ void __blk_mq_tag_idle(struct blk_mq_hw_ctx *hctx)
 	if (!test_and_clear_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
 		return;
 
-	atomic_dec(&tags->active_queues);
+	if (!blk_queue_admin(hctx->queue))
+		atomic_dec(&tags->active_queues);
 
 	blk_mq_tag_wakeup_all(tags, false);
 }
-- 
2.9.5