linux-kernel - Re: [PATCH] Revert "block: don't reorder requests in blk_add_rq_to

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Message-ID: <ed33ffb7-31bf-490c-b1ae-304a6e4b9a0f@kernel.dk>
Date: Wed, 11 Jun 2025 06:56:55 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Hazem Mohamed Abuelfotoh <abuehaze@...zon.com>
Cc: stable@...r.kernel.org, kernel test robot <oliver.sang@...el.com>,
 Hagar Hemdan <hagarhem@...zon.com>, Shaoying Xu <shaoyi@...zon.com>,
 "Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>,
 Paolo Bonzini <pbonzini@...hat.com>, Stefan Hajnoczi <stefanha@...hat.com>,
 Eugenio Pérez <eperezma@...hat.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Keith Busch <kbusch@...nel.org>,
 Christoph Hellwig <hch@....de>, Sagi Grimberg <sagi@...mberg.me>,
 linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 virtualization@...ts.linux.dev, linux-nvme@...ts.infradead.org
Subject: Re: [PATCH] Revert "block: don't reorder requests in
 blk_add_rq_to_plug"

On 6/11/25 6:14 AM, Hazem Mohamed Abuelfotoh wrote:
> This reverts commit e70c301faece15b618e54b613b1fd6ece3dd05b4.
> 
> Commit <e70c301faece> ("block: don't reorder requests in
> blk_add_rq_to_plug") reversed how requests are stored in the blk_plug
> list, this had significant impact on bio merging with requests exist on
> the plug list. This impact has been reported in [1] and could easily be
> reproducible using 4k randwrite fio benchmark on an NVME based SSD without
> having any filesystem on the disk.

Rather than revert this commit, why not just attempt a tail merge?
Something ala this, totally untested.

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 3af1d284add5..708ded67d52a 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -998,6 +998,10 @@ bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
 	if (!plug || rq_list_empty(&plug->mq_list))
 		return false;

+	rq = plug->mq_list.tail;
+	if (rq->q == q)
+		return blk_attempt_bio_merge(q, rq, bio, nr_segs, false) == BIO_MERGE_OK;
+
 	rq_list_for_each(&plug->mq_list, rq) {
 		if (rq->q == q) {
 			if (blk_attempt_bio_merge(q, rq, bio, nr_segs, false) ==

-- 
Jens Axboe