blk-mq: make plug work for mutiple disks and queues
authorShaohua Li <shli@fb.com>
Fri, 8 May 2015 17:51:33 +0000 (10:51 -0700)
committerJens Axboe <axboe@fb.com>
Fri, 8 May 2015 20:17:23 +0000 (14:17 -0600)
Last patch makes plug work for multiple queue case. However it only
works for single disk case, because it assumes only one request in the
plug list. If a task is accessing multiple disks, eg MD/DM, the
assumption is wrong. Let blk_attempt_plug_merge() record request from
the same queue.

V2: use NULL parameter in !mq case. Fix a bug. Add comments in
blk_attempt_plug_merge to make it less (hopefully) confusion.

Cc: Jens Axboe <axboe@fb.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
block/blk-core.c
block/blk-mq.c
block/blk.h

index 9dcfb8ec554b196f455762bd3abb7bf8f4ddbf26..f0be754c7781e9b45fc7208177eeb5b8bd94de16 100644 (file)
@@ -1522,7 +1522,8 @@ bool bio_attempt_front_merge(struct request_queue *q, struct request *req,
  * Caller must ensure !blk_queue_nomerges(q) beforehand.
  */
 bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
-                           unsigned int *request_count)
+                           unsigned int *request_count,
+                           struct request **same_queue_rq)
 {
        struct blk_plug *plug;
        struct request *rq;
@@ -1542,8 +1543,16 @@ bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
        list_for_each_entry_reverse(rq, plug_list, queuelist) {
                int el_ret;
 
-               if (rq->q == q)
+               if (rq->q == q) {
                        (*request_count)++;
+                       /*
+                        * Only blk-mq multiple hardware queues case checks the
+                        * rq in the same queue, there should be only one such
+                        * rq in a queue
+                        **/
+                       if (same_queue_rq)
+                               *same_queue_rq = rq;
+               }
 
                if (rq->q != q || !blk_rq_merge_ok(rq, bio))
                        continue;
@@ -1608,7 +1617,7 @@ void blk_queue_bio(struct request_queue *q, struct bio *bio)
         * any locks.
         */
        if (!blk_queue_nomerges(q) &&
-           blk_attempt_plug_merge(q, bio, &request_count))
+           blk_attempt_plug_merge(q, bio, &request_count, NULL))
                return;
 
        spin_lock_irq(q->queue_lock);
index 902c2eb9a0e74edddbe1f075cd123eadda659099..31df474436997e9adc1865f1955b72d89dba862d 100644 (file)
@@ -1269,6 +1269,7 @@ static void blk_mq_make_request(struct request_queue *q, struct bio *bio)
        struct request *rq;
        unsigned int request_count = 0;
        struct blk_plug *plug;
+       struct request *same_queue_rq = NULL;
 
        blk_queue_bounce(q, &bio);
 
@@ -1278,7 +1279,7 @@ static void blk_mq_make_request(struct request_queue *q, struct bio *bio)
        }
 
        if (!is_flush_fua && !blk_queue_nomerges(q) &&
-           blk_attempt_plug_merge(q, bio, &request_count))
+           blk_attempt_plug_merge(q, bio, &request_count, &same_queue_rq))
                return;
 
        rq = blk_mq_map_request(q, bio, &data);
@@ -1309,9 +1310,12 @@ static void blk_mq_make_request(struct request_queue *q, struct bio *bio)
                 * issued. So the plug list will have one request at most
                 */
                if (plug) {
-                       if (!list_empty(&plug->mq_list)) {
-                               old_rq = list_first_entry(&plug->mq_list,
-                                       struct request, queuelist);
+                       /*
+                        * The plug list might get flushed before this. If that
+                        * happens, same_queue_rq is invalid and plug list is empty
+                        **/
+                       if (same_queue_rq && !list_empty(&plug->mq_list)) {
+                               old_rq = same_queue_rq;
                                list_del_init(&old_rq->queuelist);
                        }
                        list_add_tail(&rq->queuelist, &plug->mq_list);
@@ -1360,7 +1364,7 @@ static void blk_sq_make_request(struct request_queue *q, struct bio *bio)
        }
 
        if (!is_flush_fua && !blk_queue_nomerges(q) &&
-           blk_attempt_plug_merge(q, bio, &request_count))
+           blk_attempt_plug_merge(q, bio, &request_count, NULL))
                return;
 
        rq = blk_mq_map_request(q, bio, &data);
index 4b48d55e588effaf2b47d1679af46634ca68768d..026d9594142bdedf1d5a0b919390bb54b3f857c1 100644 (file)
@@ -78,7 +78,8 @@ bool bio_attempt_front_merge(struct request_queue *q, struct request *req,
 bool bio_attempt_back_merge(struct request_queue *q, struct request *req,
                            struct bio *bio);
 bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
-                           unsigned int *request_count);
+                           unsigned int *request_count,
+                           struct request **same_queue_rq);
 
 void blk_account_io_start(struct request *req, bool new_io);
 void blk_account_io_completion(struct request *req, unsigned int bytes);