[PATCH 3/5] nvmet: Reorder fields in 'struct nvmf_ctrl_options'

Christophe JAILLET
2023-05-01T15:11:56Z

Group some variables based on their sizes to reduce holes.
On x86_64, this shrinks the size of 'struct nvmf_ctrl_options' from 136 to
128 bytes.

When such a structure is allocated in nvmf_create_ctrl(), because of the
way memory allocation works, when 136 bytes were requested, 192 bytes were
allocated.

So this saves 64 bytes per allocation, 1 cache line to hold the whole
structure and a few cycles when zeroing the memory in nvmf_create_ctrl().

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
---
Using pahole

Before:
======
struct nvmf_ctrl_options {
unsigned int mask; /* 0 4 */

/* XXX 4 bytes hole, try to pack */

char * transport; /* 8 8 */
char * subsysnqn; /* 16 8 */
char * traddr; /* 24 8 */
char * trsvcid; /* 32 8 */
char * host_traddr; /* 40 8 */
char * host_iface; /* 48 8 */
size_t queue_size; /* 56 8 */
/* --- cacheline 1 boundary (64 bytes) --- */
unsigned int nr_io_queues; /* 64 4 */
unsigned int reconnect_delay; /* 68 4 */
bool discovery_nqn; /* 72 1 */
bool duplicate_connect; /* 73 1 */

/* XXX 2 bytes hole, try to pack */

unsigned int kato; /* 76 4 */
struct nvmf_host * host; /* 80 8 */
int max_reconnects; /* 88 4 */

/* XXX 4 bytes hole, try to pack */

char * dhchap_secret; /* 96 8 */
char * dhchap_ctrl_secret; /* 104 8 */
bool disable_sqflow; /* 112 1 */
bool hdr_digest; /* 113 1 */
bool data_digest; /* 114 1 */

/* XXX 1 byte hole, try to pack */

unsigned int nr_write_queues; /* 116 4 */
unsigned int nr_poll_queues; /* 120 4 */
int tos; /* 124 4 */
/* --- cacheline 2 boundary (128 bytes) --- */
int fast_io_fail_tmo; /* 128 4 */

/* size: 136, cachelines: 3, members: 24 */
/* sum members: 121, holes: 4, sum holes: 11 */
/* padding: 4 */
/* last cacheline: 8 bytes */
};

After:
=====
struct nvmf_ctrl_options {
unsigned int mask; /* 0 4 */
int max_reconnects; /* 4 4 */
char * transport; /* 8 8 */
char * subsysnqn; /* 16 8 */
char * traddr; /* 24 8 */
char * trsvcid; /* 32 8 */
char * host_traddr; /* 40 8 */
char * host_iface; /* 48 8 */
size_t queue_size; /* 56 8 */
/* --- cacheline 1 boundary (64 bytes) --- */
unsigned int nr_io_queues; /* 64 4 */
unsigned int reconnect_delay; /* 68 4 */
bool discovery_nqn; /* 72 1 */
bool duplicate_connect; /* 73 1 */

/* XXX 2 bytes hole, try to pack */

unsigned int kato; /* 76 4 */
struct nvmf_host * host; /* 80 8 */
char * dhchap_secret; /* 88 8 */
char * dhchap_ctrl_secret; /* 96 8 */
bool disable_sqflow; /* 104 1 */
bool hdr_digest; /* 105 1 */
bool data_digest; /* 106 1 */

/* XXX 1 byte hole, try to pack */

unsigned int nr_write_queues; /* 108 4 */
unsigned int nr_poll_queues; /* 112 4 */
int tos; /* 116 4 */
int fast_io_fail_tmo; /* 120 4 */

/* size: 128, cachelines: 2, members: 24 */
/* sum members: 121, holes: 2, sum holes: 3 */
/* padding: 4 */
};
---
drivers/nvme/host/fabrics.h | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/nvme/host/fabrics.h b/drivers/nvme/host/fabrics.h
index dcac3df8a5f7..1bc57fb75825 100644
--- a/drivers/nvme/host/fabrics.h
+++ b/drivers/nvme/host/fabrics.h
@@ -77,6 +77,9 @@ enum {
* with the parsing opts enum.
* @mask: Used by the fabrics library to parse through sysfs options
* on adding a NVMe controller.
+ * @max_reconnects: maximum number of allowed reconnect attempts before removing
+ * the controller, (-1) means reconnect forever, zero means remove
+ * immediately;
* @transport: Holds the fabric transport "technology name" (for a lack of
* better description) that will be used by an NVMe controller
* being added.
@@ -96,9 +99,6 @@ enum {
* @discovery_nqn: indicates if the subsysnqn is the well-known discovery NQN.
* @kato: Keep-alive timeout.
* @host: Virtual NVMe host, contains the NQN and Host ID.
- * @max_reconnects: maximum number of allowed reconnect attempts before removing
- * the controller, (-1) means reconnect forever, zero means remove
- * immediately;
* @dhchap_secret: DH-HMAC-CHAP secret
* @dhchap_ctrl_secret: DH-HMAC-CHAP controller secret for bi-directional
* authentication
@@ -112,6 +112,7 @@ enum {
*/
struct nvmf_ctrl_options {
unsigned mask;
+ int max_reconnects;
char *transport;
char *subsysnqn;
char *traddr;
@@ -125,7 +126,6 @@ struct nvmf_ctrl_options {
bool duplicate_connect;
unsigned int kato;
struct nvmf_host *host;
- int max_reconnects;
char *dhchap_secret;
char *dhchap_ctrl_secret;
bool disable_sqflow;
--
2.34.1