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@wanadoo.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

在 2025/08/02 15:25, Christophe JAILLET 写道:
> 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@wanadoo.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(-)
> 

LGTM
Reviewed-by: Yu Kuai <yukuai3@huawei.com>

> 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;
> 

On Sat, 02 Aug 2025 09:25:59 +0200, Christophe JAILLET wrote:
> 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.
> 
> [...]

Applied, thanks!

[1/1] block, bfq: Reorder struct bfq_iocq_bfqq_data
      commit: 407728da41cd6450cec6a4277027015a75744d56

Best regards,
-- 
Jens Axboe