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-rw-r--r--block/blk-mq-sched.c22
1 files changed, 22 insertions, 0 deletions
diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 856356b1619e..74cedea56034 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -398,6 +398,28 @@ void blk_mq_sched_insert_request(struct request *rq, bool at_head,
WARN_ON(e && (rq->tag != -1));
if (blk_mq_sched_bypass_insert(hctx, !!e, rq)) {
+ /*
+ * Firstly normal IO request is inserted to scheduler queue or
+ * sw queue, meantime we add flush request to dispatch queue(
+ * hctx->dispatch) directly and there is at most one in-flight
+ * flush request for each hw queue, so it doesn't matter to add
+ * flush request to tail or front of the dispatch queue.
+ *
+ * Secondly in case of NCQ, flush request belongs to non-NCQ
+ * command, and queueing it will fail when there is any
+ * in-flight normal IO request(NCQ command). When adding flush
+ * rq to the front of hctx->dispatch, it is easier to introduce
+ * extra time to flush rq's latency because of S_SCHED_RESTART
+ * compared with adding to the tail of dispatch queue, then
+ * chance of flush merge is increased, and less flush requests
+ * will be issued to controller. It is observed that ~10% time
+ * is saved in blktests block/004 on disk attached to AHCI/NCQ
+ * drive when adding flush rq to the front of hctx->dispatch.
+ *
+ * Simply queue flush rq to the front of hctx->dispatch so that
+ * intensive flush workloads can benefit in case of NCQ HW.
+ */
+ at_head = (rq->rq_flags & RQF_FLUSH_SEQ) ? true : at_head;
blk_mq_request_bypass_insert(rq, at_head, false);
goto run;
}