Commit ebf05c7d authored by Greg Kroah-Hartman's avatar Greg Kroah-Hartman
Browse files

tty: shrink the size of struct tty_struct by 40 bytes 

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>
Link: https://lore.kernel.org/r/2023082519-cobbler-unholy-8d1f@gregkh


Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent a84853c5

include/linux/tty.h

+9 −9
Original line number Diff line number Diff line
@@ -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 */