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, 1 insertions, 1 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) {