[PATCH] kobject: Reorder fields in 'struct kobject'

[Christophe JAILLET]

Group some variables based on their sizes to reduce hole and avoid padding.
On x86_64, this shrinks the size of 'struct kobject' from 256 to 244 bytes.

This structure is often included in some other structures. So these other
structures will also benefit from this 8 bytes saving.

This is especially nice for structure like 'cma_kobject' or 'class_dir'
that are now 256 bytes long. When they are kzalloc()'ed, 256 bytes are
allocated, instead of 512.

Signed-off-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>
---
Try to see how often this is included in another struct. Certainly not
perfect, but gives an idea.
   git grep -P \\tstruct\ kobj[^*]*$ | wc -l
   163


Using pahole

Before:
======
struct kobject {
	const char  *              name;                 /*     0     8 */
	struct list_head           entry;                /*     8    16 */
	struct kobject *           parent;               /*    24     8 */
	struct kset *              kset;                 /*    32     8 */
	const struct kobj_type  *  ktype;                /*    40     8 */
	struct kernfs_node *       sd;                   /*    48     8 */
	struct kref                kref;                 /*    56     4 */

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

	/* --- cacheline 1 boundary (64 bytes) --- */
	struct delayed_work        release;              /*    64   184 */

	/* XXX last struct has 4 bytes of padding */

	/* --- cacheline 3 boundary (192 bytes) was 56 bytes ago --- */
	unsigned int               state_initialized:1;  /*   248: 0  4 */
	unsigned int               state_in_sysfs:1;     /*   248: 1  4 */
	unsigned int               state_add_uevent_sent:1; /*   248: 2  4 */
	unsigned int               state_remove_uevent_sent:1; /*   248: 3  4 */
	unsigned int               uevent_suppress:1;    /*   248: 4  4 */

	/* size: 256, cachelines: 4, members: 13 */
	/* sum members: 244, holes: 1, sum holes: 4 */
	/* sum bitfield members: 5 bits (0 bytes) */
	/* padding: 4 */
	/* paddings: 1, sum paddings: 4 */
	/* bit_padding: 27 bits */
};


After:
=====
struct kobject {
	const char  *              name;                 /*     0     8 */
	struct list_head           entry;                /*     8    16 */
	struct kobject *           parent;               /*    24     8 */
	struct kset *              kset;                 /*    32     8 */
	const struct kobj_type  *  ktype;                /*    40     8 */
	struct kernfs_node *       sd;                   /*    48     8 */
	struct kref                kref;                 /*    56     4 */
	unsigned int               state_initialized:1;  /*    60: 0  4 */
	unsigned int               state_in_sysfs:1;     /*    60: 1  4 */
	unsigned int               state_add_uevent_sent:1; /*    60: 2  4 */
	unsigned int               state_remove_uevent_sent:1; /*    60: 3  4 */
	unsigned int               uevent_suppress:1;    /*    60: 4  4 */

	/* XXX 27 bits hole, try to pack */

	/* --- cacheline 1 boundary (64 bytes) --- */
	struct delayed_work        release;              /*    64   184 */

	/* XXX last struct has 4 bytes of padding */

	/* size: 248, cachelines: 4, members: 13 */
	/* sum members: 244 */
	/* sum bitfield members: 5 bits, bit holes: 1, sum bit holes: 27 bits */
	/* paddings: 1, sum paddings: 4 */
	/* last cacheline: 56 bytes */
};
---
 include/linux/kobject.h | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/include/linux/kobject.h b/include/linux/kobject.h
index c392c811d9ad..c30affcc43b4 100644
--- a/include/linux/kobject.h
+++ b/include/linux/kobject.h
@@ -69,14 +69,16 @@ struct kobject {
 	const struct kobj_type	*ktype;
 	struct kernfs_node	*sd; /* sysfs directory entry */
 	struct kref		kref;
-#ifdef CONFIG_DEBUG_KOBJECT_RELEASE
-	struct delayed_work	release;
-#endif
+
 	unsigned int state_initialized:1;
 	unsigned int state_in_sysfs:1;
 	unsigned int state_add_uevent_sent:1;
 	unsigned int state_remove_uevent_sent:1;
 	unsigned int uevent_suppress:1;
+
+#ifdef CONFIG_DEBUG_KOBJECT_RELEASE
+	struct delayed_work	release;
+#endif
 };
 
 __printf(2, 3) int kobject_set_name(struct kobject *kobj, const char *name, ...);
-- 
2.34.1