linux-kernel - [PATCH] lockdep: optimise "struct lock

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Date:   Thu, 16 Nov 2023 14:25:35 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH] lockdep: optimise "struct lock_class" layout

struct lock_class does

	const struct lock_trace *	usage_traces[];
	int				name_version;
	const char *			name;

which wastes 4 bytes after "name_version".

Put pointer after pointers shrinking sizeof from 208 bytes to 200 bytes.
Space savings are considerable for such a trivial patch:

	$ ./scripts/bloat-o-meter ../vmlinux-000 ../obj/vmlinux
	add/remove: 0/0 grow/shrink: 1/11 up/down: 2/-65640 (-65638)
	Function                       old     new   delta
	check_irq_usage               2852    2854      +2
	reacquire_held_locks           486     484      -2
	lock_chain_get_class            41      39      -2
	l_start                         34      32      -2
	check_noncircular              276     274      -2
	print_usage_bug.part           688     683      -5
	print_circular_bug            1017    1011      -6
	hlock_conflict                 141     135      -6
	print_deadlock_bug             944     935      -9
	mark_lock                     3791    3769     -22
	__lock_acquire                9801    9753     -48
	lock_classes               1703936 1638400  -65536
	Total: Before=21163908, After=21098270, chg -0.31%

Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
---

 include/linux/lockdep_types.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- 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;