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>
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 7e53038..b355b59 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1172,7 +1172,12 @@
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,