[PATCH 1/7] block: improve logic around when to sort a plug list

[Jens Axboe]

Only do it if we have requests for multiple queues in the same
plug.

Signed-off-by: Jens Axboe <ax...@kernel.dk>
---
 block/blk-core.c       |  1 +
 block/blk-mq.c         | 23 ++++++++++++++++++-----
 include/linux/blkdev.h |  1 +
 3 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index be9233400314..d107d016b92b 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1780,6 +1780,7 @@ void blk_start_plug(struct blk_plug *plug)
        INIT_LIST_HEAD(&plug->mq_list);
        INIT_LIST_HEAD(&plug->cb_list);
        plug->rq_count = 0;
+       plug->multiple_queues = false;
 
        /*
         * Store ordering should not be needed here, since a potential
diff --git a/block/blk-mq.c b/block/blk-mq.c
index a82830f39933..0e2663048d95 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1677,7 +1677,8 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool 
from_schedule)
        list_splice_init(&plug->mq_list, &list);
        plug->rq_count = 0;
 
-       list_sort(NULL, &list, plug_rq_cmp);
+       if (plug->rq_count > 2 && plug->multiple_queues)
+               list_sort(NULL, &list, plug_rq_cmp);
 
        this_q = NULL;
        this_hctx = NULL;
@@ -1866,6 +1867,20 @@ void blk_mq_try_issue_list_directly(struct blk_mq_hw_ctx 
*hctx,
        }
 }
 
+static void blk_add_rq_to_plug(struct blk_plug *plug, struct request *rq)
+{
+       list_add_tail(&rq->queuelist, &plug->mq_list);
+       plug->rq_count++;
+       if (!plug->multiple_queues && !list_is_singular(&plug->mq_list)) {
+               struct request *tmp;
+
+               tmp = list_first_entry(&plug->mq_list, struct request,
+                                               queuelist);
+               if (tmp->q != rq->q)
+                       plug->multiple_queues = true;
+       }
+}
+
 static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 {
        const int is_sync = op_is_sync(bio->bi_opf);
@@ -1932,8 +1947,7 @@ static blk_qc_t blk_mq_make_request(struct request_queue 
*q, struct bio *bio)
                        trace_block_plug(q);
                }
 
-               list_add_tail(&rq->queuelist, &plug->mq_list);
-               plug->rq_count++;
+               blk_add_rq_to_plug(plug, rq);
        } else if (plug && !blk_queue_nomerges(q)) {
                blk_mq_bio_to_request(rq, bio);
 
@@ -1950,8 +1964,7 @@ static blk_qc_t blk_mq_make_request(struct request_queue 
*q, struct bio *bio)
                        list_del_init(&same_queue_rq->queuelist);
                        plug->rq_count--;
                }
-               list_add_tail(&rq->queuelist, &plug->mq_list);
-               plug->rq_count++;
+               blk_add_rq_to_plug(plug, rq);
 
                blk_mq_put_ctx(data.ctx);
 
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 02732cae6080..08d940f85fa0 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1131,6 +1131,7 @@ struct blk_plug {
        struct list_head mq_list; /* blk-mq requests */
        struct list_head cb_list; /* md requires an unplug callback */
        unsigned short rq_count;
+       bool multiple_queues;
 };
 #define BLK_MAX_REQUEST_COUNT 16
 #define BLK_PLUG_FLUSH_SIZE (128 * 1024)
-- 
2.17.1