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>

1 files changed, 6 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,