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 <ming.lei@xxxxxxxxxx>
---
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
