blk-mq: add BLK_MQ_F_DEFER_ISSUE support flag

Drivers can now tell blk-mq if they take advantage of the deferred
issue through 'last' or not. If they do, don't do queue-direct
for sync IO. This is a preparation patch for the nvme conversion.

Signed-off-by: Jens Axboe <axboe@fb.com>

2 files changed, 7 insertions, 1 deletions

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 7e5303820452..b355b5957cd7 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1172,7 +1172,12 @@ static void blk_mq_make_request(struct request_queue *q, struct bio *bio)
                 goto run_queue;
         }
 
-        if (is_sync) {
+        /*
+         * If the driver supports defer issued based on 'last', then
+         * queue it up like normal since we can potentially save some
+         * CPU this way.
+         */
+        if (is_sync && !(data.hctx->flags & BLK_MQ_F_DEFER_ISSUE)) {
                 struct blk_mq_queue_data bd = {
                         .rq = rq,
                         .list = NULL,
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index be01d7a687d4..c3b64ec5321e 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -146,6 +146,7 @@ enum {
         BLK_MQ_F_TAG_SHARED     = 1 << 1,
         BLK_MQ_F_SG_MERGE       = 1 << 2,
         BLK_MQ_F_SYSFS_UP       = 1 << 3,
+        BLK_MQ_F_DEFER_ISSUE    = 1 << 4,
 
         BLK_MQ_S_STOPPED        = 0,
         BLK_MQ_S_TAG_ACTIVE     = 1,