Re: [PATCH v1 1/1] nvme: complete directly for hctx with only one ctx mapping

[Keith Busch]

On Tue, May 30, 2023 at 10:41:19AM +0800, Po-Wen Kao wrote:
> ---
>  block/blk-mq.c           | 8 +++-----
>  drivers/nvme/host/nvme.h | 4 ++++
>  2 files changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 1749f5890606..b60c78f5ad46 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1181,12 +1181,10 @@ bool blk_mq_complete_request_remote(struct request *rq)
>  	WRITE_ONCE(rq->state, MQ_RQ_COMPLETE);
>  
>  	/*
> -	 * For request which hctx has only one ctx mapping,
> -	 * or a polled request, always complete locally,
> -	 * it's pointless to redirect the completion.
> +	 * For a polled request, always complete locally, it's pointless
> +	 * to redirect the completion.
>  	 */
> -	if (rq->mq_hctx->nr_ctx == 1 ||
> -		rq->cmd_flags & REQ_POLLED)
> +	if (rq->cmd_flags & REQ_POLLED)
>  		return false;
>  
>  	if (blk_mq_complete_need_ipi(rq)) {
> diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
> index 7cf8e44d135e..acc9b1ce071d 100644
> --- a/drivers/nvme/host/nvme.h
> +++ b/drivers/nvme/host/nvme.h
> @@ -702,6 +702,10 @@ static inline bool nvme_try_complete_req(struct request *req, __le16 status,
>  	nvme_should_fail(req);
>  	if (unlikely(blk_should_fake_timeout(req->q)))
>  		return true;
> +	if (likely(req->mq_hctx->nr_ctx == 1)) {
> +		WRITE_ONCE(req->state, MQ_RQ_COMPLETE);
> +		return false;
> +	}

I don't think we want low level drivers directly messing with blk-mq
request state.

Is the early nr_ctx check optimisation really worth it? Would the
following work for your use case?

---
diff --git a/block/blk-mq.c b/block/blk-mq.c
index f6dad0886a2fa..a2d65bb127e29 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1176,7 +1176,8 @@ bool blk_mq_complete_request_remote(struct request *rq)
         * or a polled request, always complete locally,
         * it's pointless to redirect the completion.
         */
-       if (rq->mq_hctx->nr_ctx == 1 ||
+       if ((rq->mq_hctx->nr_ctx == 1 &&
+            rq->mq_ctx->cpu == raw_smp_processor_id()) ||
                rq->cmd_flags & REQ_POLLED)
                return false;
--