block: attempt to merge with existing requests on plug flush

One of the disadvantages of on-stack plugging is that we potentially
lose out on merging since all pending IO isn't always visible to
everybody. When we flush the on-stack plugs, right now we don't do
any checks to see if potential merge candidates could be utilized.

Correct this by adding a new insert variant, ELEVATOR_INSERT_SORT_MERGE.
It works just ELEVATOR_INSERT_SORT, but first checks whether we can
merge with an existing request before doing the insertion (if we fail
merging).

This fixes a regression with multiple processes issuing IO that
can be merged.

Thanks to Shaohua Li <shaohua.li@intel.com> for testing and fixing
an accounting bug.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

1 files changed, 49 insertions, 3 deletions

diff --git a/block/elevator.c b/block/elevator.c
index 542ce826b401..c387d3168734 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -521,6 +521,40 @@ int elv_merge(struct request_queue *q, struct request **req, struct bio *bio)
         return ELEVATOR_NO_MERGE;
 }
 
+/*
+ * Attempt to do an insertion back merge. Only check for the case where
+ * we can append 'rq' to an existing request, so we can throw 'rq' away
+ * afterwards.
+ *
+ * Returns true if we merged, false otherwise
+ */
+static bool elv_attempt_insert_merge(struct request_queue *q,
+                                     struct request *rq)
+{
+        struct request *__rq;
+
+        if (blk_queue_nomerges(q))
+                return false;
+
+        /*
+         * First try one-hit cache.
+         */
+        if (q->last_merge && blk_attempt_req_merge(q, q->last_merge, rq))
+                return true;
+
+        if (blk_queue_noxmerges(q))
+                return false;
+
+        /*
+         * See if our hash lookup can find a potential backmerge.
+         */
+        __rq = elv_rqhash_find(q, blk_rq_pos(rq));
+        if (__rq && blk_attempt_req_merge(q, __rq, rq))
+                return true;
+
+        return false;
+}
+
 void elv_merged_request(struct request_queue *q, struct request *rq, int type)
 {
         struct elevator_queue *e = q->elevator;
@@ -538,14 +572,18 @@ void elv_merge_requests(struct request_queue *q, struct request *rq,
                              struct request *next)
 {
         struct elevator_queue *e = q->elevator;
+        const int next_sorted = next->cmd_flags & REQ_SORTED;
 
-        if (e->ops->elevator_merge_req_fn)
+        if (next_sorted && e->ops->elevator_merge_req_fn)
                 e->ops->elevator_merge_req_fn(q, rq, next);
 
         elv_rqhash_reposition(q, rq);
-        elv_rqhash_del(q, next);
 
-        q->nr_sorted--;
+        if (next_sorted) {
+                elv_rqhash_del(q, next);
+                q->nr_sorted--;
+        }
+
         q->last_merge = rq;
 }
 
@@ -647,6 +685,14 @@ void elv_insert(struct request_queue *q, struct request *rq, int where)
                 __blk_run_queue(q, false);
                 break;
 
+        case ELEVATOR_INSERT_SORT_MERGE:
+                /*
+                 * If we succeed in merging this request with one in the
+                 * queue already, we are done - rq has now been freed,
+                 * so no need to do anything further.
+                 */
+                if (elv_attempt_insert_merge(q, rq))
+                        break;
         case ELEVATOR_INSERT_SORT:
                 BUG_ON(rq->cmd_type != REQ_TYPE_FS &&
                        !(rq->cmd_flags & REQ_DISCARD));