MIPS: Optimize spinlocks.

The current locking mechanism uses a ll/sc sequence to release a
spinlock.  This is slower than a wmb() followed by a store to unlock.

The branching forward to .subsection 2 on sc failure slows down the
contended case.  So we get rid of that part too.

Since we are now working on naturally aligned u16 values, we can get
rid of a masking operation as the LHU already does the right thing.
The ANDI are reversed for better scheduling on multi-issue CPUs

On a 12 CPU 750MHz Octeon cn5750 this patch improves ipv4 UDP packet
forwarding rates from 3.58*10^6 PPS to 3.99*10^6 PPS, or about 11%.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/937/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 18 insertions, 6 deletions

diff --git a/arch/mips/include/asm/spinlock_types.h b/arch/mips/include/asm/spinlock_types.h
index ee197c2f9c98..c52f36013a9d 100644
--- a/arch/mips/include/asm/spinlock_types.h
+++ b/arch/mips/include/asm/spinlock_types.h
@@ -5,16 +5,28 @@
 # error "please don't include this file directly"
 #endif
 
-typedef struct {
+#include <linux/types.h>
+
+#include <asm/byteorder.h>
+
+typedef union {
 	/*
-	 * bits  0..13: serving_now
-	 * bits 14    : junk data
-	 * bits 15..28: ticket
+	 * bits  0..15 : serving_now
+	 * bits 16..31 : ticket
 	 */
-	unsigned int lock;
+	u32 lock;
+	struct {
+#ifdef __BIG_ENDIAN
+		u16 ticket;
+		u16 serving_now;
+#else
+		u16 serving_now;
+		u16 ticket;
+#endif
+	} h;
 } arch_spinlock_t;
 
-#define __ARCH_SPIN_LOCK_UNLOCKED	{ 0 }
+#define __ARCH_SPIN_LOCK_UNLOCKED	{ .lock = 0 }
 
 typedef struct {
 	volatile unsigned int lock;