[tip: locking/core] locking/lockdep: Slightly reorder 'struct lock_class' to save some memory

[tip-bot2 for Christophe JAILLET]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     18caaedaf4c3712ab6821f292598a8f86e6d7972
Gitweb:        https://git.kernel.org/tip/18caaedaf4c3712ab6821f292598a8f86e6d7972
Author:        Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>
AuthorDate:    Sun, 26 Nov 2023 09:56:29 +01:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Mon, 27 Nov 2023 09:46:52 +01:00

locking/lockdep: Slightly reorder 'struct lock_class' to save some memory

Based on pahole, 2 holes can be combined in the 'struct lock_class'. This
saves 8 bytes in the structure on my x86_64.

On a x86_64 configured with allmodconfig, this saves ~64kb of memory in
'kernel/locking/lockdep.o':

                text         data           bss           dec     filename
  Before:    102,501    1,912,490    11,531,636    13,546,627     kernel/locking/lockdep.o
  After:     102,181    1,912,490    11,466,100    13,480,771     kernel/locking/lockdep.o

because of:

  struct lock_class lock_classes[MAX_LOCKDEP_KEYS];

After the reorder, pahole gives:

  struct lock_class {
          struct hlist_node          hash_entry;           /*     0    16 */
          struct list_head           lock_entry;           /*    16    16 */
          struct list_head           locks_after;          /*    32    16 */
          struct list_head           locks_before;         /*    48    16 */
          /* --- cacheline 1 boundary (64 bytes) --- */
          const struct lockdep_subclass_key  * key;        /*    64     8 */
          lock_cmp_fn                cmp_fn;               /*    72     8 */
          lock_print_fn              print_fn;             /*    80     8 */
          unsigned int               subclass;             /*    88     4 */
          unsigned int               dep_gen_id;           /*    92     4 */
          long unsigned int          usage_mask;           /*    96     8 */
          const struct lock_trace  * usage_traces[10];     /*   104    80 */
          /* --- cacheline 2 boundary (128 bytes) was 56 bytes ago --- */
          const char  *              name;                 /*   184     8 */
          /* --- cacheline 3 boundary (192 bytes) --- */
          int                        name_version;         /*   192     4 */
          u8                         wait_type_inner;      /*   196     1 */
          u8                         wait_type_outer;      /*   197     1 */
          u8                         lock_type;            /*   198     1 */

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

          long unsigned int          contention_point[4];  /*   200    32 */
          long unsigned int          contending_point[4];  /*   232    32 */

          /* size: 264, cachelines: 5, members: 18 */
          /* sum members: 263, holes: 1, sum holes: 1 */
          /* last cacheline: 8 bytes */
  };

Signed-off-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Acked-by: Waiman Long <longman@xxxxxxxxxx>
Link: https://lore.kernel.org/r/801258371fc4101f96495a5aaecef638d6cbd8d3.1700988869.git.christophe.jaillet@xxxxxxxxxx
---
 include/linux/lockdep_types.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/lockdep_types.h b/include/linux/lockdep_types.h
index 2ebc323..857d785 100644
--- a/include/linux/lockdep_types.h
+++ b/include/linux/lockdep_types.h
@@ -127,12 +127,12 @@ struct lock_class {
 	unsigned long			usage_mask;
 	const struct lock_trace		*usage_traces[LOCK_TRACE_STATES];
 
+	const char			*name;
 	/*
 	 * Generation counter, when doing certain classes of graph walking,
 	 * to ensure that we check one node only once:
 	 */
 	int				name_version;
-	const char			*name;
 
 	u8				wait_type_inner;
 	u8				wait_type_outer;