linux-kernel - [RFC PATCH] blk-mq: hctx has only one ctx mapping is no need to redirect the completion

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Message-Id: <1663731123-81536-1-git-send-email-liusong@linux.alibaba.com>
Date:   Wed, 21 Sep 2022 11:32:03 +0800
From:   Liu Song <liusong@...ux.alibaba.com>
To:     axboe@...nel.dk
Cc:     hch@....de, kbusch@...nel.org, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, liusong@...ux.alibaba.com,
        sagi@...mberg.me
Subject: [RFC PATCH] blk-mq: hctx has only one ctx mapping is no need to redirect the completion

From: Liu Song <liusong@...ux.alibaba.com>

High-performance NVMe devices usually support a large hw queue, which
ensures a 1:1 mapping of hctx and ctx. In this case there will be no
remote request, so we don't need to care about it.

Signed-off-by: Liu Song <liusong@...ux.alibaba.com>
---
 block/blk-mq.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index c11949d..9626ea1 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1093,10 +1093,12 @@ bool blk_mq_complete_request_remote(struct request *rq)
 	WRITE_ONCE(rq->state, MQ_RQ_COMPLETE);
 
 	/*
-	 * For a polled request, always complete locally, it's pointless
-	 * to redirect the completion.
+	 * For request which hctx has only one ctx mapping,
+	 * or a polled request, always complete locally,
+	 * it's pointless to redirect the completion.
 	 */
-	if (rq->cmd_flags & REQ_POLLED)
+	if (rq->mq_hctx->nr_ctx == 1 ||
+		rq->cmd_flags & REQ_POLLED)
 		return false;
 
 	if (blk_mq_complete_need_ipi(rq)) {
-- 
1.8.3.1