linux-kernel - [tip:perfcounters/urgent] x86: atomic64: The atomic64_t data type should be 8 bytes aligned on 32-bit too

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Date:	Fri, 3 Jul 2009 12:42:34 GMT
From:	tip-bot for Eric Dumazet <eric.dumazet@...il.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, acme@...hat.com, paulus@...ba.org,
	hpa@...or.com, mingo@...hat.com, eric.dumazet@...il.com,
	a.p.zijlstra@...llo.nl, torvalds@...ux-foundation.org,
	efault@....de, arnd@...db.de, dhowells@...hat.com,
	fweisbec@...il.com, akpm@...ux-foundation.org, tglx@...utronix.de,
	mingo@...e.hu
Subject: [tip:perfcounters/urgent] x86: atomic64: The atomic64_t data type should be 8 bytes aligned on 32-bit too

Commit-ID:  bbf2a330d92c5afccfd17592ba9ccd50f41cf748
Gitweb:     http://git.kernel.org/tip/bbf2a330d92c5afccfd17592ba9ccd50f41cf748
Author:     Eric Dumazet <eric.dumazet@...il.com>
AuthorDate: Fri, 3 Jul 2009 00:08:26 +0200
Committer:  Ingo Molnar <mingo@...e.hu>
CommitDate: Fri, 3 Jul 2009 13:26:38 +0200

x86: atomic64: The atomic64_t data type should be 8 bytes aligned on 32-bit too

Locked instructions on two cache lines at once are painful. If
atomic64_t uses two cache lines, my test program is 10x slower.

The chance for that is significant: 4/32 or 12.5%.

Make sure an atomic64_t is 8 bytes aligned.

Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: Mike Galbraith <efault@....de>
Cc: Paul Mackerras <paulus@...ba.org>
Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: David Howells <dhowells@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Arnd Bergmann <arnd@...db.de>
LKML-Reference: <alpine.LFD.2.01.0907021653030.3210@...alhost.localdomain>
[ changed it to __aligned(8) as per Andrew's suggestion ]
Signed-off-by: Ingo Molnar <mingo@...e.hu>


---
 arch/x86/include/asm/atomic_32.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/x86/include/asm/atomic_32.h b/arch/x86/include/asm/atomic_32.h
index 2503d4e..ae0fbb5 100644
--- a/arch/x86/include/asm/atomic_32.h
+++ b/arch/x86/include/asm/atomic_32.h
@@ -250,7 +250,7 @@ static inline int atomic_add_unless(atomic_t *v, int a, int u)
 /* An 64bit atomic type */
 
 typedef struct {
-	unsigned long long counter;
+	unsigned long long __aligned(8) counter;
 } atomic64_t;
 
 #define ATOMIC64_INIT(val)	{ (val) }
--
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