path: root/block/blk-core.c
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/blk-core.c
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/blk-core.c')
1 files changed, 1 insertions, 2 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);

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