Before blk-mq is introduced, I/O is merged before putting into plug queue, but blk-mq changed the order and makes merging basically impossible until mq-deadline is introduced. Then it is observed that throughput of sequential I/O is degraded about 10%~20% on virtio-blk in the test[1] if IO schedluer isn't used.
This patch provides a default per-sw-queue bio merging if there isn't scheduler enabled or the scheduler hasn't implement .bio_merge(), and this way actually moves merging before plugging just like what blk_queue_bio() does, then the performance regression is fixed. [1]. test script: sudo fio --direct=1 --size=128G --bsrange=4k-4k --runtime=40 --numjobs=16 --ioengine=libaio --iodepth=64 --group_reporting=1 --filename=/dev/vdb --name=virtio_blk-test-$RW --rw=$RW --output-format=json RW=read or write Signed-off-by: Ming Lei <[email protected]> --- block/blk-mq-sched.c | 61 +++++++++++++++++++++++++++++++++---- block/blk-mq-sched.h | 4 +-- block/blk-mq.c | 85 +++++++--------------------------------------------- 3 files changed, 66 insertions(+), 84 deletions(-) diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c index 1f5b692526ae..ad1754e370d1 100644 --- a/block/blk-mq-sched.c +++ b/block/blk-mq-sched.c @@ -221,19 +221,68 @@ bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio, } EXPORT_SYMBOL_GPL(blk_mq_sched_try_merge); +/* + * Reverse check our software queue for entries that we could potentially + * merge with. Currently includes a hand-wavy stop count of 8, to not spend + * too much time checking for merges. + */ +static bool blk_mq_attempt_merge(struct request_queue *q, + struct blk_mq_ctx *ctx, struct bio *bio) +{ + struct request *rq; + int checked = 8; + + list_for_each_entry_reverse(rq, &ctx->rq_list, queuelist) { + bool merged = false; + + if (!checked--) + break; + + if (!blk_rq_merge_ok(rq, bio)) + continue; + + switch (blk_try_merge(rq, bio)) { + case ELEVATOR_BACK_MERGE: + if (blk_mq_sched_allow_merge(q, rq, bio)) + merged = bio_attempt_back_merge(q, rq, bio); + break; + case ELEVATOR_FRONT_MERGE: + if (blk_mq_sched_allow_merge(q, rq, bio)) + merged = bio_attempt_front_merge(q, rq, bio); + break; + case ELEVATOR_DISCARD_MERGE: + merged = bio_attempt_discard_merge(q, rq, bio); + break; + default: + continue; + } + + if (merged) + ctx->rq_merged++; + return merged; + } + + return false; +} + bool __blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio) { struct elevator_queue *e = q->elevator; + struct blk_mq_ctx *ctx = blk_mq_get_ctx(q); + struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, ctx->cpu); + bool ret = false; - if (e->type->ops.mq.bio_merge) { - struct blk_mq_ctx *ctx = blk_mq_get_ctx(q); - struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, ctx->cpu); - + if (e && e->type->ops.mq.bio_merge) { blk_mq_put_ctx(ctx); return e->type->ops.mq.bio_merge(hctx, bio); + } else if (hctx->flags & BLK_MQ_F_SHOULD_MERGE) { + /* default per sw-queue merge */ + spin_lock(&ctx->lock); + ret = blk_mq_attempt_merge(q, ctx, bio); + spin_unlock(&ctx->lock); } - - return false; + blk_mq_put_ctx(ctx); + return ret; } bool blk_mq_sched_try_insert_merge(struct request_queue *q, struct request *rq) diff --git a/block/blk-mq-sched.h b/block/blk-mq-sched.h index edafb5383b7b..b87e5be5db8c 100644 --- a/block/blk-mq-sched.h +++ b/block/blk-mq-sched.h @@ -38,9 +38,7 @@ int blk_mq_sched_init(struct request_queue *q); static inline bool blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio) { - struct elevator_queue *e = q->elevator; - - if (!e || blk_queue_nomerges(q) || !bio_mergeable(bio)) + if (blk_queue_nomerges(q) || !bio_mergeable(bio)) return false; return __blk_mq_sched_bio_merge(q, bio); diff --git a/block/blk-mq.c b/block/blk-mq.c index a69ad122ed66..6cfce2076583 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -772,50 +772,6 @@ static void blk_mq_timeout_work(struct work_struct *work) blk_queue_exit(q); } -/* - * Reverse check our software queue for entries that we could potentially - * merge with. Currently includes a hand-wavy stop count of 8, to not spend - * too much time checking for merges. - */ -static bool blk_mq_attempt_merge(struct request_queue *q, - struct blk_mq_ctx *ctx, struct bio *bio) -{ - struct request *rq; - int checked = 8; - - list_for_each_entry_reverse(rq, &ctx->rq_list, queuelist) { - bool merged = false; - - if (!checked--) - break; - - if (!blk_rq_merge_ok(rq, bio)) - continue; - - switch (blk_try_merge(rq, bio)) { - case ELEVATOR_BACK_MERGE: - if (blk_mq_sched_allow_merge(q, rq, bio)) - merged = bio_attempt_back_merge(q, rq, bio); - break; - case ELEVATOR_FRONT_MERGE: - if (blk_mq_sched_allow_merge(q, rq, bio)) - merged = bio_attempt_front_merge(q, rq, bio); - break; - case ELEVATOR_DISCARD_MERGE: - merged = bio_attempt_discard_merge(q, rq, bio); - break; - default: - continue; - } - - if (merged) - ctx->rq_merged++; - return merged; - } - - return false; -} - struct flush_busy_ctx_data { struct blk_mq_hw_ctx *hctx; struct list_head *list; @@ -1440,36 +1396,15 @@ static void blk_mq_bio_to_request(struct request *rq, struct bio *bio) blk_account_io_start(rq, true); } -static inline bool hctx_allow_merges(struct blk_mq_hw_ctx *hctx) +static inline void blk_mq_queue_io(struct blk_mq_hw_ctx *hctx, + struct blk_mq_ctx *ctx, + struct request *rq, struct bio *bio) { - return (hctx->flags & BLK_MQ_F_SHOULD_MERGE) && - !blk_queue_nomerges(hctx->queue); -} - -static inline bool blk_mq_merge_queue_io(struct blk_mq_hw_ctx *hctx, - struct blk_mq_ctx *ctx, - struct request *rq, struct bio *bio) -{ - if (!hctx_allow_merges(hctx) || !bio_mergeable(bio)) { - blk_mq_bio_to_request(rq, bio); - spin_lock(&ctx->lock); -insert_rq: - __blk_mq_insert_request(hctx, rq, false); - spin_unlock(&ctx->lock); - return false; - } else { - struct request_queue *q = hctx->queue; + blk_mq_bio_to_request(rq, bio); - spin_lock(&ctx->lock); - if (!blk_mq_attempt_merge(q, ctx, bio)) { - blk_mq_bio_to_request(rq, bio); - goto insert_rq; - } - - spin_unlock(&ctx->lock); - __blk_mq_finish_request(hctx, ctx, rq); - return true; - } + spin_lock(&ctx->lock); + __blk_mq_insert_request(hctx, rq, false); + spin_unlock(&ctx->lock); } static blk_qc_t request_to_qc_t(struct blk_mq_hw_ctx *hctx, struct request *rq) @@ -1649,11 +1584,11 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio) blk_mq_put_ctx(data.ctx); blk_mq_bio_to_request(rq, bio); blk_mq_sched_insert_request(rq, false, true, true, true); - } else if (!blk_mq_merge_queue_io(data.hctx, data.ctx, rq, bio)) { + } else { + blk_mq_queue_io(data.hctx, data.ctx, rq, bio); blk_mq_put_ctx(data.ctx); blk_mq_run_hw_queue(data.hctx, true); - } else - blk_mq_put_ctx(data.ctx); + } return cookie; } -- 2.9.3
