On 1/16/18 9:33 PM, Mike Snitzer wrote:
> From: Ming Lei <ming.lei@xxxxxxxxxx>
>
> 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).
This looks better. Two minor nit picks:
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index c117c2baf2c9..0b64f7210a89 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1706,7 +1706,8 @@ static blk_status_t __blk_mq_issue_directly(struct blk_mq_hw_ctx *hctx,
> blk_qc_t new_cookie;
> blk_status_t ret;
>
> - new_cookie = request_to_qc_t(hctx, rq);
> + if (cookie)
> + new_cookie = request_to_qc_t(hctx, rq);
>
> /*
> * For OK queue, we are done. For error, caller may kill it.
> @@ -1716,13 +1717,15 @@ static blk_status_t __blk_mq_issue_directly(struct blk_mq_hw_ctx *hctx,
> ret = q->mq_ops->queue_rq(hctx, &bd);
> switch (ret) {
> case BLK_STS_OK:
> - *cookie = new_cookie;
> + if (cookie)
> + *cookie = new_cookie;
> break;
> case BLK_STS_RESOURCE:
> __blk_mq_requeue_request(rq);
> break;
> default:
> - *cookie = BLK_QC_T_NONE;
> + if (cookie)
> + *cookie = BLK_QC_T_NONE;
> break;
> }
Instead of adding these three conditions, always pass in a valid pointer
to a cookie and get rid of them.
> @@ -1731,15 +1734,20 @@ 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)
> {
> + if (bypass_insert) {
> + blk_mq_request_bypass_insert(rq, run_queue);
> + return;
> + }
> blk_mq_sched_insert_request(rq, false, run_queue, false,
> hctx->flags & BLK_MQ_F_BLOCKING);
> }
Lose the return and just make it an if/else.
--
Jens Axboe
