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>

diff --git a/block/blk-core.c b/block/blk-core.c
index e1fcf7a24668132d3f4e4e3e811d47052af10a76..525693237a4aa7fa6c085d5410ff4e7f030a01f9 100644 (file)
--- 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 ea85e20d5e9462be965f3f117bd7c3efcf51aa57..cfcc37cb222b63722bc048814c0f392205d1a898 100644 (file)
--- 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 49d21af81d07faf36924deb463400dce2f97d6cb..c8db371a921d55760e2d9494c25a0f52c16bd659 100644 (file)
--- 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 542ce826b401a7874ac7205afd9f2641b3b6520b..c387d3168734c21d74336b536819d35dc77719a5 100644 (file)
--- 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));

diff --git a/include/linux/elevator.h b/include/linux/elevator.h
index ec6f72b844774b7414f69e99a48243ddbbc72af1..d93efcc4457050ffa1e511a825c273f60796ed69 100644 (file)
--- a/include/linux/elevator.h
+++ b/include/linux/elevator.h
@@ -166,6 +166,7 @@ extern struct request *elv_rb_find(struct rb_root *, sector_t);
 #define ELEVATOR_INSERT_SORT   3
 #define ELEVATOR_INSERT_REQUEUE        4
 #define ELEVATOR_INSERT_FLUSH  5
+#define ELEVATOR_INSERT_SORT_MERGE     6
 
 /*
  * return values from elevator_may_queue_fn