linux-kernel - [PATCH] tty: shrink the size of struct tty

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Date:   Fri, 25 Aug 2023 17:28:20 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-serial@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>
Subject: [PATCH] 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@...nel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 include/linux/tty.h | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

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 */
-- 
2.42.0