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

[Jianpeng Ma]

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 <majianp...@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;
 
-- 
1.7.9.5