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Message-Id: <6f249f9b02a3490283ef0278096556de41aa0cf0.1676626130.git.christophe.jaillet@wanadoo.fr>
Date: Fri, 17 Feb 2023 10:29:10 +0100
From: Christophe JAILLET <christophe.jaillet@...adoo.fr>
To: 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] blk-mq: Reorder fields in 'struct blk_mq_tag_set'
Group some variables based on their sizes to reduce hole and avoid padding.
On x86_64, this shrinks the size of 'struct blk_mq_tag_set'
from 304 to 296 bytes.
Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
---
Using pahole
Before:
======
struct blk_mq_tag_set {
struct blk_mq_queue_map map[3]; /* 0 48 */
unsigned int nr_maps; /* 48 4 */
/* XXX 4 bytes hole, try to pack */
const struct blk_mq_ops * ops; /* 56 8 */
/* --- cacheline 1 boundary (64 bytes) --- */
unsigned int nr_hw_queues; /* 64 4 */
unsigned int queue_depth; /* 68 4 */
unsigned int reserved_tags; /* 72 4 */
unsigned int cmd_size; /* 76 4 */
int numa_node; /* 80 4 */
unsigned int timeout; /* 84 4 */
unsigned int flags; /* 88 4 */
/* XXX 4 bytes hole, try to pack */
void * driver_data; /* 96 8 */
struct blk_mq_tags * * tags; /* 104 8 */
struct blk_mq_tags * shared_tags; /* 112 8 */
struct mutex tag_list_lock; /* 120 160 */
/* --- cacheline 4 boundary (256 bytes) was 24 bytes ago --- */
struct list_head tag_list; /* 280 16 */
struct srcu_struct * srcu; /* 296 8 */
/* size: 304, cachelines: 5, members: 16 */
/* sum members: 296, holes: 2, sum holes: 8 */
/* last cacheline: 48 bytes */
};
After:
=====
struct blk_mq_tag_set {
const struct blk_mq_ops * ops; /* 0 8 */
struct blk_mq_queue_map map[3]; /* 8 48 */
unsigned int nr_maps; /* 56 4 */
unsigned int nr_hw_queues; /* 60 4 */
/* --- cacheline 1 boundary (64 bytes) --- */
unsigned int queue_depth; /* 64 4 */
unsigned int reserved_tags; /* 68 4 */
unsigned int cmd_size; /* 72 4 */
int numa_node; /* 76 4 */
unsigned int timeout; /* 80 4 */
unsigned int flags; /* 84 4 */
void * driver_data; /* 88 8 */
struct blk_mq_tags * * tags; /* 96 8 */
struct blk_mq_tags * shared_tags; /* 104 8 */
struct mutex tag_list_lock; /* 112 160 */
/* --- cacheline 4 boundary (256 bytes) was 16 bytes ago --- */
struct list_head tag_list; /* 272 16 */
struct srcu_struct * srcu; /* 288 8 */
/* size: 296, cachelines: 5, members: 16 */
/* last cacheline: 40 bytes */
};
---
include/linux/blk-mq.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 779fba613bd0..dd5ce1137f04 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -473,6 +473,7 @@ enum hctx_type {
/**
* struct blk_mq_tag_set - tag set that can be shared between request queues
+ * @ops: Pointers to functions that implement block driver behavior.
* @map: One or more ctx -> hctx mappings. One map exists for each
* hardware queue type (enum hctx_type) that the driver wishes
* to support. There are no restrictions on maps being of the
@@ -480,7 +481,6 @@ enum hctx_type {
* types.
* @nr_maps: Number of elements in the @map array. A number in the range
* [1, HCTX_MAX_TYPES].
- * @ops: Pointers to functions that implement block driver behavior.
* @nr_hw_queues: Number of hardware queues supported by the block driver that
* owns this data structure.
* @queue_depth: Number of tags per hardware queue, reserved tags included.
@@ -505,9 +505,9 @@ enum hctx_type {
* (BLK_MQ_F_BLOCKING).
*/
struct blk_mq_tag_set {
+ const struct blk_mq_ops *ops;
struct blk_mq_queue_map map[HCTX_MAX_TYPES];
unsigned int nr_maps;
- const struct blk_mq_ops *ops;
unsigned int nr_hw_queues;
unsigned int queue_depth;
unsigned int reserved_tags;
--
2.34.1
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