path: root/block
diff options
authorMing Lei <ming.lei@redhat.com>2018-01-17 11:25:57 -0500
committerJens Axboe <axboe@kernel.dk>2018-01-17 09:46:54 -0700
commit396eaf21ee17c476e8f66249fb1f4a39003d0ab4 (patch)
tree20b146db746cc279c734fd937128c45cb8ed5c76 /block
parent0f95549c0ea1e8075ae049202088b2c6a0cb40ad (diff)
blk-mq: improve DM's blk-mq IO merging via blk_insert_cloned_request feedback
blk_insert_cloned_request() is called in the fast path of a dm-rq driver (e.g. blk-mq request-based DM mpath). blk_insert_cloned_request() uses blk_mq_request_bypass_insert() to directly append the request to the blk-mq hctx->dispatch_list of the underlying queue. 1) This way isn't efficient enough because the hctx spinlock is always used. 2) With blk_insert_cloned_request(), we completely bypass underlying queue's elevator and depend on the upper-level dm-rq driver's elevator to schedule IO. But dm-rq currently can't get the underlying queue's dispatch feedback at all. Without knowing whether a request was issued or not (e.g. due to underlying queue being busy) the dm-rq elevator will not be able to provide effective IO merging (as a side-effect of dm-rq currently blindly destaging a request from its elevator only to requeue it after a delay, which kills any opportunity for merging). This obviously causes very bad sequential IO performance. Fix this by updating blk_insert_cloned_request() to use blk_mq_request_direct_issue(). blk_mq_request_direct_issue() allows a request to be issued directly to the underlying queue and returns the dispatch feedback (blk_status_t). If blk_mq_request_direct_issue() returns BLK_SYS_RESOURCE the dm-rq driver will now use DM_MAPIO_REQUEUE to _not_ destage the request. Whereby preserving the opportunity to merge IO. With this, request-based DM's blk-mq sequential IO performance is vastly improved (as much as 3X in mpath/virtio-scsi testing). Signed-off-by: Ming Lei <ming.lei@redhat.com> [blk-mq.c changes heavily influenced by Ming Lei's initial solution, but they were refactored to make them less fragile and easier to read/review] Signed-off-by: Mike Snitzer <snitzer@redhat.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
Diffstat (limited to 'block')
3 files changed, 33 insertions, 10 deletions
diff --git a/block/blk-core.c b/block/blk-core.c
index 7ba607527487..55f338020254 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2500,8 +2500,7 @@ blk_status_t blk_insert_cloned_request(struct request_queue *q, struct request *
* bypass a potential scheduler on the bottom device for
* insert.
- blk_mq_request_bypass_insert(rq, true);
- return BLK_STS_OK;
+ return blk_mq_request_direct_issue(rq);
spin_lock_irqsave(q->queue_lock, flags);
diff --git a/block/blk-mq.c b/block/blk-mq.c
index ddc46f215bfa..e383a20809f4 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1775,15 +1775,19 @@ static blk_status_t __blk_mq_issue_directly(struct blk_mq_hw_ctx *hctx,
static void __blk_mq_fallback_to_insert(struct blk_mq_hw_ctx *hctx,
struct request *rq,
- bool run_queue)
+ bool run_queue, bool bypass_insert)
- blk_mq_sched_insert_request(rq, false, run_queue, false,
- hctx->flags & BLK_MQ_F_BLOCKING);
+ if (!bypass_insert)
+ blk_mq_sched_insert_request(rq, false, run_queue, false,
+ hctx->flags & BLK_MQ_F_BLOCKING);
+ else
+ blk_mq_request_bypass_insert(rq, run_queue);
static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
struct request *rq,
- blk_qc_t *cookie)
+ blk_qc_t *cookie,
+ bool bypass_insert)
struct request_queue *q = rq->q;
bool run_queue = true;
@@ -1794,7 +1798,7 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
goto insert;
- if (q->elevator)
+ if (q->elevator && !bypass_insert)
goto insert;
if (!blk_mq_get_driver_tag(rq, NULL, false))
@@ -1807,7 +1811,9 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
return __blk_mq_issue_directly(hctx, rq, cookie);
- __blk_mq_fallback_to_insert(hctx, rq, run_queue);
+ __blk_mq_fallback_to_insert(hctx, rq, run_queue, bypass_insert);
+ if (bypass_insert)
return BLK_STS_OK;
@@ -1822,15 +1828,30 @@ static void blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
hctx_lock(hctx, &srcu_idx);
- ret = __blk_mq_try_issue_directly(hctx, rq, cookie);
+ ret = __blk_mq_try_issue_directly(hctx, rq, cookie, false);
if (ret == BLK_STS_RESOURCE)
- __blk_mq_fallback_to_insert(hctx, rq, true);
+ __blk_mq_fallback_to_insert(hctx, rq, true, false);
else if (ret != BLK_STS_OK)
blk_mq_end_request(rq, ret);
hctx_unlock(hctx, srcu_idx);
+blk_status_t blk_mq_request_direct_issue(struct request *rq)
+ blk_status_t ret;
+ int srcu_idx;
+ blk_qc_t unused_cookie;
+ struct blk_mq_ctx *ctx = rq->mq_ctx;
+ struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(rq->q, ctx->cpu);
+ hctx_lock(hctx, &srcu_idx);
+ ret = __blk_mq_try_issue_directly(hctx, rq, &unused_cookie, true);
+ hctx_unlock(hctx, srcu_idx);
+ return ret;
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);
diff --git a/block/blk-mq.h b/block/blk-mq.h
index 8591a54d989b..e3ebc93646ca 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -74,6 +74,9 @@ void blk_mq_request_bypass_insert(struct request *rq, bool run_queue);
void blk_mq_insert_requests(struct blk_mq_hw_ctx *hctx, struct blk_mq_ctx *ctx,
struct list_head *list);
+/* Used by blk_insert_cloned_request() to issue request directly */
+blk_status_t blk_mq_request_direct_issue(struct request *rq);
* CPU -> queue mappings

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