linux-kernel - [PATCH] block: plug attempts to batch allocate tags multiple times

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Message-Id: <20250901082209.13349-1-xue01.he@samsung.com>
Date: Mon,  1 Sep 2025 08:22:09 +0000
From: Xue He <xue01.he@...sung.com>
To: axboe@...nel.dk
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org, hexue
	<xue01.he@...sung.com>
Subject: [PATCH] block: plug attempts to batch allocate tags multiple times

From: hexue <xue01.he@...sung.com>

In the existing plug mechanism, tags are allocated in batches based on
the number of requests. However, testing has shown that the plug only
attempts batch allocation of tags once at the beginning of a batch of
I/O operations. Since the tag_mask does not always have enough available
tags to satisfy the requested number, a full batch allocation is not
guaranteed to succeed each time. The remaining tags are then allocated
individually (occurs frequently), leading to multiple single-tag
allocation overheads.

This patch aims to allow the remaining I/O operations to retry batch
allocation of tags, reducing the overhead caused by multiple
individual tag allocations.

------------------------------------------------------------------------
test result
During testing of the PCIe Gen4 SSD Samsung PM9A3, the perf tool
observed CPU improvements. The CPU usage of the original function
_blk_mq_alloc_requests function was 1.39%, which decreased to 0.82%
after modification.

Additionally, performance variations were observed on different devices.
workload:randread
blocksize:4k
thread:1
------------------------------------------------------------------------
                  PCIe Gen3 SSD   PCIe Gen4 SSD    PCIe Gen5 SSD
native kernel     553k iops       633k iops        793k iops
modified          553k iops       635k iops        801k iops

with Optane SSDs, the performance like
two device one thread
cmd :sudo taskset -c 0 ./t/io_uring -b512 -d128 -c32 -s32 -p1 -F1 -B1
-n1 -r4 /dev/nvme0n1 /dev/nvme1n1

base: 6.4 Million IOPS
patch: 6.49 Million IOPS

two device two thread
cmd: sudo taskset -c 0 ./t/io_uring -b512 -d128 -c32 -s32 -p1 -F1 -B1
-n1 -r4 /dev/nvme0n1 /dev/nvme1n1

base: 7.34 Million IOPS
patch: 7.48 Million IOPS
-------------------------------------------------------------------------

Signed-off-by: hexue <xue01.he@...sung.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 b67d6c02eceb..1fb280764b76 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -587,9 +587,9 @@ static struct request *blk_mq_rq_cache_fill(struct request_queue *q,
 	if (blk_queue_enter(q, flags))
 		return NULL;
 
-	plug->nr_ios = 1;
-
 	rq = __blk_mq_alloc_requests(&data);
+	plug->nr_ios = data.nr_tags;
+
 	if (unlikely(!rq))
 		blk_queue_exit(q);
 	return rq;
@@ -3034,11 +3034,13 @@ static struct request *blk_mq_get_new_requests(struct request_queue *q,
 
 	if (plug) {
 		data.nr_tags = plug->nr_ios;
-		plug->nr_ios = 1;
 		data.cached_rqs = &plug->cached_rqs;
 	}
 
 	rq = __blk_mq_alloc_requests(&data);
+	if (plug)
+		plug->nr_ios = data.nr_tags;
+
 	if (unlikely(!rq))
 		rq_qos_cleanup(q, bio);
 	return rq;
-- 
2.34.1