[RFC PATCH] block:Fix some problems about handling plug in blk_queue_bio().

["Jianpeng Ma" <majianpeng@gmail.com>]

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I think there are three problems about handling plug in blk_queue_bio():
1:if request_count >= BLK_MAX_REQUEST_COUNT, avoid unnecessary plug->should_sort judge.
2:Only two device can trace plug.
3:When exec blk_flush_plug_list,it use list_sort which has
O(nlog(n)) complexity. When insert and sort, it only O(n) complexity.

Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
---
 block/blk-core.c |   32 +++++++++++++++-----------------
 1 file changed, 15 insertions(+), 17 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 4b4dbdf..e7759f8 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1514,20 +1514,31 @@ get_rq:
 		if (list_empty(&plug->list))
 			trace_block_plug(q);
 		else {
-			if (!plug->should_sort) {
+			if (request_count >= BLK_MAX_REQUEST_COUNT) {
+				blk_flush_plug_list(plug, false);
+				trace_block_plug(q);
+			} else	if (!plug->should_sort) {
 				struct request *__rq;
 
 				__rq = list_entry_rq(plug->list.prev);
 				if (__rq->q != q)
 					plug->should_sort = 1;
-			}
-			if (request_count >= BLK_MAX_REQUEST_COUNT) {
-				blk_flush_plug_list(plug, false);
+			} else  {
+				struct request *rq;
+
+				list_for_each_entry_reverse(rq, &plug->list, queuelist) {
+					if (rq->q == q) {
+						list_add(&req->queuelist, &rq->queuelist);
+						goto stat_acct;
+					}
+				}
 				trace_block_plug(q);
 			}
 		}
 		list_add_tail(&req->queuelist, &plug->list);
+stat_acct:
 		drive_stat_acct(req, 1);
+
 	} else {
 		spin_lock_irq(q->queue_lock);
 		add_acct_request(q, req, where);
@@ -2866,14 +2877,6 @@ void blk_start_plug(struct blk_plug *plug)
 }
 EXPORT_SYMBOL(blk_start_plug);
 
-static int plug_rq_cmp(void *priv, struct list_head *a, struct list_head *b)
-{
-	struct request *rqa = container_of(a, struct request, queuelist);
-	struct request *rqb = container_of(b, struct request, queuelist);
-
-	return !(rqa->q <= rqb->q);
-}
-
 /*
  * If 'from_schedule' is true, then postpone the dispatch of requests
  * until a safe kblockd context. We due this to avoid accidental big
@@ -2967,11 +2970,6 @@ void blk_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 
 	list_splice_init(&plug->list, &list);
 
-	if (plug->should_sort) {
-		list_sort(NULL, &list, plug_rq_cmp);
-		plug->should_sort = 0;
-	}
-
 	q = NULL;
 	depth = 0;
 
-- 
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