linux-kernel - [PATCH] lockdep: slightly reorder 'struct lock

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Message-Id: <801258371fc4101f96495a5aaecef638d6cbd8d3.1700988869.git.christophe.jaillet@wanadoo.fr>
Date:   Sun, 26 Nov 2023 09:56:29 +0100
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     peterz@...radead.org, mingo@...hat.com, will@...nel.org,
        longman@...hat.com, boqun.feng@...il.com
Cc:     linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>
Subject: [PATCH] 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', certainly because of:
  struct lock_class lock_classes[MAX_LOCKDEP_KEYS];

Before:
   text	   data	    bss	    dec	    hex	filename
 102501	1912490	11531636	13546627	 ceb483	kernel/locking/lockdep.o

After:
 102181	1912490	11466100	13480771	 cdb343	kernel/locking/lockdep.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
---
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 */
};
---
 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 2ebc323d345a..857d785e89e6 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;
-- 
2.34.1