mm/memcontrol.c: make mem_cgroup_id_get_many() __maybe_unused

[linux-2.6-microblaze.git] / block / blk-mq-sched.c

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 856356b..74cedea 100644 (file)
--- 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;
        }