linux-kernel - [PATCH] block, bfq: Reorder struct bfq_iocq_bfqq

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Message-ID: <79394db1befaa658e8066b8e3348073ce27d9d26.1754119538.git.christophe.jaillet@wanadoo.fr>
Date: Sat,  2 Aug 2025 09:25:59 +0200
From: Christophe JAILLET <christophe.jaillet@...adoo.fr>
To: Yu Kuai <yukuai3@...wei.com>,
	Jens Axboe <axboe@...nel.dk>
Cc: linux-kernel@...r.kernel.org,
	kernel-janitors@...r.kernel.org,
	Christophe JAILLET <christophe.jaillet@...adoo.fr>,
	linux-block@...r.kernel.org
Subject: [PATCH] block, bfq: Reorder struct bfq_iocq_bfqq_data

The size of struct bfq_iocq_bfqq_data can be reduced by moving a few fields
around.

On a x86_64, with allmodconfig, this shrinks the size from 144 to 128
bytes.
The main benefit is to reduce the size of struct bfq_io_cq from 1360 to
1232.

This structure is stored in a dedicated slab cache. So reducing its size
improves cache usage.

Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
---
Compile tested only.

On my system, struct bfq_io_cq are stored in 8 pages slab. Each of these
slabs hold 24 entries.

$ sudo cat /proc/slabinfo | grep bfq_io
bfq_io_cq            378    384   1360   24    8 : tunables    0    0    0 : slabdata     16     16      0

With the new layout, we should store 26 entries.
(8 * 4096 / 1232 = 26.60)
---
 block/bfq-iosched.h | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
index 687a3a7ba784..0b4704932d72 100644
--- a/block/bfq-iosched.h
+++ b/block/bfq-iosched.h
@@ -427,9 +427,6 @@ struct bfq_iocq_bfqq_data {
 	 */
 	bool saved_IO_bound;
 
-	u64 saved_io_start_time;
-	u64 saved_tot_idle_time;
-
 	/*
 	 * Same purpose as the previous fields for the values of the
 	 * field keeping the queue's belonging to a large burst
@@ -450,6 +447,9 @@ struct bfq_iocq_bfqq_data {
 	 */
 	unsigned int saved_weight;
 
+	u64 saved_io_start_time;
+	u64 saved_tot_idle_time;
+
 	/*
 	 * Similar to previous fields: save wr information.
 	 */
@@ -457,13 +457,13 @@ struct bfq_iocq_bfqq_data {
 	unsigned long saved_last_wr_start_finish;
 	unsigned long saved_service_from_wr;
 	unsigned long saved_wr_start_at_switch_to_srt;
-	unsigned int saved_wr_cur_max_time;
 	struct bfq_ttime saved_ttime;
+	unsigned int saved_wr_cur_max_time;
 
 	/* Save also injection state */
-	u64 saved_last_serv_time_ns;
 	unsigned int saved_inject_limit;
 	unsigned long saved_decrease_time_jif;
+	u64 saved_last_serv_time_ns;
 
 	/* candidate queue for a stable merge (due to close creation time) */
 	struct bfq_queue *stable_merge_bfqq;
-- 
2.50.1