linux-kernel - [PATCH RESEND v4 7/7] blk-mq: add documentation for new queue attribute async

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Message-Id: <20251116032044.118664-8-yukuai@fnnas.com>
Date: Sun, 16 Nov 2025 11:20:41 +0800
From: "Yu Kuai" <yukuai@...as.com>
To: <axboe@...nel.dk>, <linux-block@...r.kernel.org>, 
	<linux-kernel@...r.kernel.org>
Cc: <yukuai@...as.com>, <nilay@...ux.ibm.com>, <bvanassche@....org>
Subject: [PATCH RESEND v4 7/7] blk-mq: add documentation for new queue attribute async_dpeth

Explain the attribute and the default value in different case.

Signed-off-by: Yu Kuai <yukuai@...as.com>
Reviewed-by: Nilay Shroff <nilay@...ux.ibm.com>
---
 Documentation/ABI/stable/sysfs-block | 34 ++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/Documentation/ABI/stable/sysfs-block b/Documentation/ABI/stable/sysfs-block
index 0ed10aeff86b..aa1e94169666 100644
--- a/Documentation/ABI/stable/sysfs-block
+++ b/Documentation/ABI/stable/sysfs-block
@@ -609,6 +609,40 @@ Description:
 		enabled, and whether tags are shared.
 
 
+What:		/sys/block/<disk>/queue/async_depth
+Date:		August 2025
+Contact:	linux-block@...r.kernel.org
+Description:
+		[RW] Controls how many asynchronous requests may be allocated in the
+		block layer. The value is always capped at nr_requests.
+
+		When no elevator is active (none):
+		- async_depth is always equal to nr_requests.
+
+		For bfq scheduler:
+		- By default, async_depth is set to 75% of nr_requests.
+		  Internal limits are then derived from this value:
+		  * Sync writes: limited to async_depth (≈75% of nr_requests).
+		  * Async I/O: limited to ~2/3 of async_depth (≈50% of nr_requests).
+
+		  If a bfq_queue is weight-raised:
+		  * Sync writes: limited to ~1/2 of async_depth (≈37% of nr_requests).
+		  * Async I/O: limited to ~1/4 of async_depth (≈18% of nr_requests).
+
+		- If the user writes a custom value to async_depth, BFQ will recompute
+		  these limits proportionally based on the new value.
+
+		For Kyber:
+		- By default async_depth is set to 75% of nr_requests.
+		- If the user writes a custom value to async_depth, then it override the
+		  default and directly control the limit for writes and async I/O.
+
+		For mq-deadline:
+		- By default async_depth is set to nr_requests.
+		- If the user writes a custom value to async_depth, then it override the
+		  default and directly control the limit for writes and async I/O.
+
+
 What:		/sys/block/<disk>/queue/nr_zones
 Date:		November 2018
 Contact:	Damien Le Moal <damien.lemoal@....com>
-- 
2.51.0