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>

4 files changed, 58 insertions, 4 deletions

diff --git a/block/blk-core.c b/block/blk-core.c
index e1fcf7a24668..525693237a4a 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2685,7 +2685,7 @@ static void flush_plug_list(struct blk_plug *plug)
                 /*
                  * rq is already accounted, so use raw insert
                  */
-                __elv_add_request(q, rq, ELEVATOR_INSERT_SORT);
+                __elv_add_request(q, rq, ELEVATOR_INSERT_SORT_MERGE);
         }
 
         if (q) {
diff --git a/block/blk-merge.c b/block/blk-merge.c
index ea85e20d5e94..cfcc37cb222b 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -465,3 +465,9 @@ int attempt_front_merge(struct request_queue *q, struct request *rq)
 
         return 0;
 }
+
+int blk_attempt_req_merge(struct request_queue *q, struct request *rq,
+                          struct request *next)
+{
+        return attempt_merge(q, rq, next);
+}
diff --git a/block/blk.h b/block/blk.h
index 49d21af81d07..c8db371a921d 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -103,6 +103,8 @@ int ll_front_merge_fn(struct request_queue *q, struct request *req,
                       struct bio *bio);
 int attempt_back_merge(struct request_queue *q, struct request *rq);
 int attempt_front_merge(struct request_queue *q, struct request *rq);
+int blk_attempt_req_merge(struct request_queue *q, struct request *rq,
+                                struct request *next);
 void blk_recalc_rq_segments(struct request *rq);
 void blk_rq_set_mixed_merge(struct request *rq);
 
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));