diff mbox series

tty: shrink the size of struct tty_struct by 40 bytes

Message ID 2023082519-cobbler-unholy-8d1f@gregkh
State New
Headers show
Series tty: shrink the size of struct tty_struct by 40 bytes | expand

Commit Message

gregkh@linuxfoundation.org Aug. 25, 2023, 3:28 p.m. UTC 
It's been a long time since anyone has looked at what struct tty_struct
looks like in memory, turns out there was a ton of holes.

So move things around a bit, change one variable (closing) from being an
int to a bool (it is only being tested for 0/1), and we end up saving 40
bytes per structure overall on x86-64 systems.

Before this patch:
	/* size: 696, cachelines: 11, members: 37 */
	/* sum members: 665, holes: 8, sum holes: 31 */
	/* forced alignments: 2, forced holes: 1, sum forced holes: 4 */
	/* last cacheline: 56 bytes */

After this change:
	/* size: 656, cachelines: 11, members: 37 */
	/* sum members: 654, holes: 1, sum holes: 2 */
	/* forced alignments: 2 */
	/* last cacheline: 16 bytes */

Cc: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 include/linux/tty.h | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

Comments

gregkh@linuxfoundation.org Aug. 25, 2023, 3:34 p.m. UTC | #1 
On Fri, Aug 25, 2023 at 05:28:20PM +0200, Greg Kroah-Hartman wrote:
>  	bool hw_stopped;
> -	unsigned int receive_room;
> +	bool closing;
>  	int flow_change;

Note, "flow_change" really only has 2 values, 1 or 2, so it could be a
boolean, or a u8 at the worse (both take the same amount of space).  But
changing that here doesn't affect the overall size of the structure,
just increases the hole at this location in the structure, so it's not a
big deal at this point in time.

thanks,

greg k-h
diff mbox series

Patch

diff --git a/include/linux/tty.h b/include/linux/tty.h
index e8d5d9997aca..f002d0f25db7 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -192,13 +192,14 @@  struct tty_operations;
  */
 struct tty_struct {
 	struct kref kref;
+	int index;
 	struct device *dev;
 	struct tty_driver *driver;
+	struct tty_port *port;
 	const struct tty_operations *ops;
-	int index;
 
-	struct ld_semaphore ldisc_sem;
 	struct tty_ldisc *ldisc;
+	struct ld_semaphore ldisc_sem;
 
 	struct mutex atomic_write_lock;
 	struct mutex legacy_mutex;
@@ -209,6 +210,7 @@  struct tty_struct {
 	char name[64];
 	unsigned long flags;
 	int count;
+	unsigned int receive_room;
 	struct winsize winsize;
 
 	struct {
@@ -219,16 +221,16 @@  struct tty_struct {
 	} __aligned(sizeof(unsigned long)) flow;
 
 	struct {
-		spinlock_t lock;
 		struct pid *pgrp;
 		struct pid *session;
+		spinlock_t lock;
 		unsigned char pktstatus;
 		bool packet;
 		unsigned long unused[0];
 	} __aligned(sizeof(unsigned long)) ctrl;
 
 	bool hw_stopped;
-	unsigned int receive_room;
+	bool closing;
 	int flow_change;
 
 	struct tty_struct *link;
@@ -239,15 +241,13 @@  struct tty_struct {
 	void *disc_data;
 	void *driver_data;
 	spinlock_t files_lock;
+	int write_cnt;
+	unsigned char *write_buf;
+
 	struct list_head tty_files;
 
 #define N_TTY_BUF_SIZE 4096
-
-	int closing;
-	unsigned char *write_buf;
-	int write_cnt;
 	struct work_struct SAK_work;
-	struct tty_port *port;
 } __randomize_layout;
 
 /* Each of a tty's open files has private_data pointing to tty_file_private */