blk-mq: simplify the plug handling in blk_mq_submit_bio

blk_mq_submit_bio has two different plug cases, one that uses full
plugging and a limited plugging one.

The limited plugging case is only used for a corner case that does
not matter in real life:

 - no ->commit_rqs (so not NVMe)
 - no shared tags (so not SCSI)
 - not rotational (so no old disk or floppy driver)
 - must have multiple queues (so no eMMC)

Remove the limited merging case and all the related junk to simplify
blk_mq_submit_bio and the functions called from it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211123160443.1315598-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 893c1a6..ba761c3 100644 (file)
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -1067,7 +1067,6 @@ static enum bio_merge_status blk_attempt_bio_merge(struct request_queue *q,
  * @q: request_queue new bio is being queued at
  * @bio: new bio being queued
  * @nr_segs: number of segments in @bio
- * @same_queue_rq: output value, will be true if there's an existing request
  * from the passed in @q already in the plug list
  *
  * Determine whether @bio being queued on @q can be merged with the previous
@@ -1084,7 +1083,7 @@ static enum bio_merge_status blk_attempt_bio_merge(struct request_queue *q,
  * Caller must ensure !blk_queue_nomerges(q) beforehand.
  */
 bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
-               unsigned int nr_segs, bool *same_queue_rq)
+               unsigned int nr_segs)
 {
        struct blk_plug *plug;
        struct request *rq;
@@ -1096,12 +1095,6 @@ bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
        /* check the previously added entry for a quick merge attempt */
        rq = rq_list_peek(&plug->mq_list);
        if (rq->q == q) {
-               /*
-                * Only blk-mq multiple hardware queues case checks the rq in
-                * the same queue, there should be only one such rq in a queue
-                */
-               *same_queue_rq = true;
-
                if (blk_attempt_bio_merge(q, rq, bio, nr_segs, false) ==
                                BIO_MERGE_OK)
                        return true;

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 00df1eb..4a13900 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2690,11 +2690,10 @@ static inline unsigned short blk_plug_max_rq_count(struct blk_plug *plug)
 }
 
 static bool blk_mq_attempt_bio_merge(struct request_queue *q,
-                                    struct bio *bio, unsigned int nr_segs,
-                                    bool *same_queue_rq)
+                                    struct bio *bio, unsigned int nr_segs)
 {
        if (!blk_queue_nomerges(q) && bio_mergeable(bio)) {
-               if (blk_attempt_plug_merge(q, bio, nr_segs, same_queue_rq))
+               if (blk_attempt_plug_merge(q, bio, nr_segs))
                        return true;
                if (blk_mq_sched_bio_merge(q, bio, nr_segs))
                        return true;
@@ -2705,8 +2704,7 @@ static bool blk_mq_attempt_bio_merge(struct request_queue *q,
 static struct request *blk_mq_get_new_requests(struct request_queue *q,
                                               struct blk_plug *plug,
                                               struct bio *bio,
-                                              unsigned int nsegs,
-                                              bool *same_queue_rq)
+                                              unsigned int nsegs)
 {
        struct blk_mq_alloc_data data = {
                .q              = q,
@@ -2715,7 +2713,7 @@ static struct request *blk_mq_get_new_requests(struct request_queue *q,
        };
        struct request *rq;
 
-       if (blk_mq_attempt_bio_merge(q, bio, nsegs, same_queue_rq))
+       if (blk_mq_attempt_bio_merge(q, bio, nsegs))
                return NULL;
 
        rq_qos_throttle(q, bio);
@@ -2751,8 +2749,7 @@ static inline bool blk_mq_can_use_cached_rq(struct request *rq, struct bio *bio)
 static inline struct request *blk_mq_get_request(struct request_queue *q,
                                                 struct blk_plug *plug,
                                                 struct bio *bio,
-                                                unsigned int nsegs,
-                                                bool *same_queue_rq)
+                                                unsigned int nsegs)
 {
        struct request *rq;
        bool checked = false;
@@ -2762,8 +2759,7 @@ static inline struct request *blk_mq_get_request(struct request_queue *q,
                if (rq && rq->q == q) {
                        if (unlikely(!submit_bio_checks(bio)))
                                return NULL;
-                       if (blk_mq_attempt_bio_merge(q, bio, nsegs,
-                                               same_queue_rq))
+                       if (blk_mq_attempt_bio_merge(q, bio, nsegs))
                                return NULL;
                        checked = true;
                        if (!blk_mq_can_use_cached_rq(rq, bio))
@@ -2781,7 +2777,7 @@ fallback:
                return NULL;
        if (unlikely(!checked && !submit_bio_checks(bio)))
                goto out_put;
-       rq = blk_mq_get_new_requests(q, plug, bio, nsegs, same_queue_rq);
+       rq = blk_mq_get_new_requests(q, plug, bio, nsegs);
        if (rq)
                return rq;
 out_put:
@@ -2808,7 +2804,6 @@ void blk_mq_submit_bio(struct bio *bio)
        const int is_sync = op_is_sync(bio->bi_opf);
        struct request *rq;
        struct blk_plug *plug;
-       bool same_queue_rq = false;
        unsigned int nr_segs = 1;
        blk_status_t ret;
 
@@ -2823,7 +2818,7 @@ void blk_mq_submit_bio(struct bio *bio)
                return;
 
        plug = blk_mq_plug(q, bio);
-       rq = blk_mq_get_request(q, plug, bio, nr_segs, &same_queue_rq);
+       rq = blk_mq_get_request(q, plug, bio, nr_segs);
        if (unlikely(!rq))
                return;
 
@@ -2846,16 +2841,7 @@ void blk_mq_submit_bio(struct bio *bio)
                return;
        }
 
-       if (plug && (q->nr_hw_queues == 1 ||
-           blk_mq_is_shared_tags(rq->mq_hctx->flags) ||
-           q->mq_ops->commit_rqs || !blk_queue_nonrot(q))) {
-               /*
-                * Use plugging if we have a ->commit_rqs() hook as well, as
-                * we know the driver uses bd->last in a smart fashion.
-                *
-                * Use normal plugging if this disk is slow HDD, as sequential
-                * IO may benefit a lot from plug merging.
-                */
+       if (plug) {
                unsigned int request_count = plug->rq_count;
                struct request *last = NULL;
 
@@ -2873,40 +2859,12 @@ void blk_mq_submit_bio(struct bio *bio)
                }
 
                blk_add_rq_to_plug(plug, rq);
-       } else if (rq->rq_flags & RQF_ELV) {
-               /* Insert the request at the IO scheduler queue */
+       } else if ((rq->rq_flags & RQF_ELV) ||
+                  (rq->mq_hctx->dispatch_busy &&
+                   (q->nr_hw_queues == 1 || !is_sync))) {
                blk_mq_sched_insert_request(rq, false, true, true);
-       } else if (plug && !blk_queue_nomerges(q)) {
-               struct request *next_rq = NULL;
-
-               /*
-                * We do limited plugging. If the bio can be merged, do that.
-                * Otherwise the existing request in the plug list will be
-                * issued. So the plug list will have one request at most
-                * The plug list might get flushed before this. If that happens,
-                * the plug list is empty, and same_queue_rq is invalid.
-                */
-               if (same_queue_rq) {
-                       next_rq = rq_list_pop(&plug->mq_list);
-                       plug->rq_count--;
-               }
-               blk_add_rq_to_plug(plug, rq);
-               trace_block_plug(q);
-
-               if (next_rq) {
-                       trace_block_unplug(q, 1, true);
-                       blk_mq_try_issue_directly(next_rq->mq_hctx, next_rq);
-               }
-       } else if ((q->nr_hw_queues > 1 && is_sync) ||
-                  !rq->mq_hctx->dispatch_busy) {
-               /*
-                * There is no scheduler and we can try to send directly
-                * to the hardware.
-                */
-               blk_mq_try_issue_directly(rq->mq_hctx, rq);
        } else {
-               /* Default case. */
-               blk_mq_sched_insert_request(rq, false, true, true);
+               blk_mq_try_issue_directly(rq->mq_hctx, rq);
        }
 }
 

diff --git a/block/blk.h b/block/blk.h
index 423cba8..5d4d08d 100644 (file)
--- a/block/blk.h
+++ b/block/blk.h
@@ -253,7 +253,7 @@ void blk_add_timer(struct request *req);
 const char *blk_status_to_str(blk_status_t status);
 
 bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
-               unsigned int nr_segs, bool *same_queue_rq);
+               unsigned int nr_segs);
 bool blk_bio_list_merge(struct request_queue *q, struct list_head *list,
                        struct bio *bio, unsigned int nr_segs);