blk-mq: make plug work for mutiple disks and queues

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>

diff --git a/block/blk-core.c b/block/blk-core.c
index 9dcfb8ec554b196f455762bd3abb7bf8f4ddbf26..f0be754c7781e9b45fc7208177eeb5b8bd94de16 100644 (file)
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -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);

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 902c2eb9a0e74edddbe1f075cd123eadda659099..31df474436997e9adc1865f1955b72d89dba862d 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -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);

diff --git a/block/blk.h b/block/blk.h
index 4b48d55e588effaf2b47d1679af46634ca68768d..026d9594142bdedf1d5a0b919390bb54b3f857c1 100644 (file)
--- a/block/blk.h
+++ b/block/blk.h
@@ -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);