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blk-mq: simplify the plug handling in blk_mq_submit_bio
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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 <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>
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Christoph Hellwig authored and axboe committed Nov 29, 2021
1 parent a4561f9 commit 0c5bcc9
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Showing 3 changed files with 15 additions and 64 deletions.
9 changes: 1 addition & 8 deletions block/blk-merge.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand All @@ -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;
Expand All @@ -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;
Expand Down
68 changes: 13 additions & 55 deletions block/blk-mq.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand All @@ -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,
Expand All @@ -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);
Expand Down Expand Up @@ -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;
Expand All @@ -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))
Expand All @@ -2781,7 +2777,7 @@ static inline struct request *blk_mq_get_request(struct request_queue *q,
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:
Expand All @@ -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;

Expand All @@ -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;

Expand All @@ -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;

Expand All @@ -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);
}
}

Expand Down
2 changes: 1 addition & 1 deletion block/blk.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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);

Expand Down

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