block: recursive merge requests

In my workload, thread 1 accesses a, a+2, ..., thread 2 accesses a+1,
a+3,.... When the requests are flushed to queue, a and a+1 are merged
to (a, a+1), a+2 and a+3 too to (a+2, a+3), but (a, a+1) and (a+2, a+3)
aren't merged.
With recursive merge below, the workload throughput gets improved 20%
and context switch drops 60%.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 12 insertions, 4 deletions

diff --git a/block/elevator.c b/block/elevator.c
index 91e18f8af9be..99838f460b44 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -515,6 +515,7 @@ static bool elv_attempt_insert_merge(struct request_queue *q,
                                      struct request *rq)
 {
         struct request *__rq;
+        bool ret;
 
         if (blk_queue_nomerges(q))
                 return false;
@@ -528,14 +529,21 @@ static bool elv_attempt_insert_merge(struct request_queue *q,
         if (blk_queue_noxmerges(q))
                 return false;
 
+        ret = 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;
+        while (1) {
+                __rq = elv_rqhash_find(q, blk_rq_pos(rq));
+                if (!__rq || !blk_attempt_req_merge(q, __rq, rq))
+                        break;
 
-        return false;
+                /* The merged request could be merged with others, try again */
+                ret = true;
+                rq = __rq;
+        }
+
+        return ret;
 }
 
 void elv_merged_request(struct request_queue *q, struct request *rq, int type)