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Subject[patch] 2.4.4 alpha semaphores optimization
Initially I tried to use __builtin_expect in the rwsem.h, but found
that it doesn't help at all in the small inline functions - it works
as expected only in a reasonably large block of code. Converting these
functions into the macros won't help, as callers are inline
functions also.
On the other hand, gcc 3.0 generates quite a good code for
conditional branches (comparisons like value < 0, value == 0
predicted as false etc.). In the cases where expected value is 0,
we can use cmpeq instruction.
Other changes:
- added atomic_add_return_prev() for __down_write()
- removed some mb's for non-SMP
- removed non-inline up()/down_xx() when semaphore/waitqueue debugging
isn't enabled.

Ivan.

--- 2.4.4/include/asm-alpha/rwsem.h Sun Feb 7 06:28:16 2106
+++ linux/include/asm-alpha/rwsem.h Thu May 3 13:01:34 2001
@@ -0,0 +1,105 @@
+#ifndef _ALPHA_RWSEM_H
+#define _ALPHA_RWSEM_H
+
+/*
+ * Written by Ivan Kokshaysky <ink@jurassic.park.msu.ru>, 2001.
+ * Based on asm-alpha/semaphore.h and asm-i386/rwsem.h
+ */
+
+#ifndef _LINUX_RWSEM_H
+#error please dont include asm/rwsem.h directly, use linux/rwsem.h instead
+#endif
+
+#ifdef __KERNEL__
+
+#include <linux/list.h>
+#include <linux/spinlock.h>
+
+struct rwsem_waiter;
+
+extern struct rw_semaphore *rwsem_down_read_failed(struct rw_semaphore *sem);
+extern struct rw_semaphore *rwsem_down_write_failed(struct rw_semaphore *sem);
+extern struct rw_semaphore *rwsem_wake(struct rw_semaphore *);
+
+/*
+ * the semaphore definition
+ */
+struct rw_semaphore {
+ atomic_t count;
+#define RWSEM_UNLOCKED_VALUE 0x00000000
+#define RWSEM_ACTIVE_BIAS 0x00000001
+#define RWSEM_ACTIVE_MASK 0x0000ffff
+#define RWSEM_WAITING_BIAS (-0x00010000)
+#define RWSEM_ACTIVE_READ_BIAS RWSEM_ACTIVE_BIAS
+#define RWSEM_ACTIVE_WRITE_BIAS (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
+ spinlock_t wait_lock;
+ struct list_head wait_list;
+#if RWSEM_DEBUG
+ int debug;
+#endif
+};
+
+#if RWSEM_DEBUG
+#define __RWSEM_DEBUG_INIT , 0
+#else
+#define __RWSEM_DEBUG_INIT /* */
+#endif
+
+#define __RWSEM_INITIALIZER(name) \
+ { ATOMIC_INIT(RWSEM_UNLOCKED_VALUE), SPIN_LOCK_UNLOCKED, \
+ LIST_HEAD_INIT((name).wait_list) __RWSEM_DEBUG_INIT }
+
+#define DECLARE_RWSEM(name) \
+ struct rw_semaphore name = __RWSEM_INITIALIZER(name)
+
+static inline void init_rwsem(struct rw_semaphore *sem)
+{
+ sem->count.counter = RWSEM_UNLOCKED_VALUE;
+ spin_lock_init(&sem->wait_lock);
+ INIT_LIST_HEAD(&sem->wait_list);
+#if RWSEM_DEBUG
+ sem->debug = 0;
+#endif
+}
+
+static inline void __down_read(struct rw_semaphore *sem)
+{
+ long count;
+ count = atomic_inc_return(&sem->count);
+ if (count < 0)
+ rwsem_down_read_failed(sem);
+}
+
+static inline void __down_write(struct rw_semaphore *sem)
+{
+ long prev, cmp;
+ prev = atomic_add_return_prev(RWSEM_ACTIVE_WRITE_BIAS, &sem->count);
+ __asm__ __volatile__("cmpeq %1,0,%0\n" : "=r" (cmp) : "r" (prev));
+ if (!cmp)
+ rwsem_down_write_failed(sem);
+}
+
+static inline void __up_read(struct rw_semaphore *sem)
+{
+ long count;
+ count = atomic_dec_return(&sem->count);
+ if (count < 0)
+ if ((count & RWSEM_ACTIVE_MASK) == 0)
+ rwsem_wake(sem);
+}
+
+static inline void __up_write(struct rw_semaphore *sem)
+{
+ long count, cmp;
+ count = atomic_sub_return(RWSEM_ACTIVE_WRITE_BIAS, &sem->count);
+ __asm__ __volatile__("cmpeq %1,0,%0\n" : "=r" (cmp) : "r" (count));
+ if (!cmp)
+ if ((count & RWSEM_ACTIVE_MASK) == 0)
+ rwsem_wake(sem);
+}
+
+#define rwsem_atomic_add(val, sem) atomic_add(val, &(sem)->count)
+#define rwsem_atomic_update(val, sem) atomic_add_return(val, &(sem)->count)
+
+#endif /* __KERNEL__ */
+#endif /* _ALPHA_RWSEM_H */
--- 2.4.4/include/asm-alpha/atomic.h Fri Apr 27 20:33:29 2001
+++ linux/include/asm-alpha/atomic.h Fri Apr 27 20:33:41 2001
@@ -70,7 +70,9 @@ static __inline__ long atomic_add_return
" addl %0,%3,%0\n"
" stl_c %0,%1\n"
" beq %0,2f\n"
+#ifdef CONFIG_SMP
" mb\n"
+#endif
".subsection 2\n"
"2: br 1b\n"
".previous"
@@ -88,11 +90,35 @@ static __inline__ long atomic_sub_return
" subl %0,%3,%0\n"
" stl_c %0,%1\n"
" beq %0,2f\n"
+#ifdef CONFIG_SMP
" mb\n"
+#endif
".subsection 2\n"
"2: br 1b\n"
".previous"
:"=&r" (temp), "=m" (v->counter), "=&r" (result)
+ :"Ir" (i), "m" (v->counter) : "memory");
+ return result;
+}
+
+/*
+ * Same as above, but return the previous value
+ */
+static __inline__ long atomic_add_return_prev(int i, atomic_t * v)
+{
+ long temp, result;
+ __asm__ __volatile__(
+ "1: ldl_l %0,%1\n"
+ " addl %0,%3,%2\n"
+ " stl_c %2,%1\n"
+ " beq %2,2f\n"
+#ifdef CONFIG_SMP
+ " mb\n"
+#endif
+ ".subsection 2\n"
+ "2: br 1b\n"
+ ".previous"
+ :"=&r" (result), "=m" (v->counter), "=&r" (temp)
:"Ir" (i), "m" (v->counter) : "memory");
return result;
}
--- 2.4.4/include/asm-alpha/semaphore.h Fri Apr 27 20:33:29 2001
+++ linux/include/asm-alpha/semaphore.h Thu May 3 13:04:47 2001
@@ -11,7 +11,6 @@
#include <asm/current.h>
#include <asm/system.h>
#include <asm/atomic.h>
-#include <linux/compiler.h>
#include <linux/wait.h>
#include <linux/rwsem.h>

@@ -92,14 +91,14 @@ extern void __up_wakeup(struct semaphore
static inline void __down(struct semaphore *sem)
{
long count = atomic_dec_return(&sem->count);
- if (__builtin_expect(count < 0, 0))
+ if (count < 0)
__down_failed(sem);
}

static inline int __down_interruptible(struct semaphore *sem)
{
long count = atomic_dec_return(&sem->count);
- if (__builtin_expect(count < 0, 0))
+ if (count < 0)
return __down_failed_interruptible(sem);
return 0;
}
@@ -201,7 +200,7 @@ static inline void __up(struct semaphore
: "m"(*sem), "r"(0x0000000100000000)
: "memory");

- if (__builtin_expect(ret <= 0, 0))
+ if (ret <= 0)
__up_wakeup(sem);
}

--- 2.4.4/arch/alpha/kernel/alpha_ksyms.c Fri Apr 27 20:33:29 2001
+++ linux/arch/alpha/kernel/alpha_ksyms.c Thu May 3 12:32:29 2001
@@ -169,14 +169,12 @@ EXPORT_SYMBOL(__strnlen_user);
EXPORT_SYMBOL(__down_failed);
EXPORT_SYMBOL(__down_failed_interruptible);
EXPORT_SYMBOL(__up_wakeup);
+#if WAITQUEUE_DEBUG || DEBUG_SEMAPHORE
EXPORT_SYMBOL(down);
EXPORT_SYMBOL(down_interruptible);
EXPORT_SYMBOL(down_trylock);
EXPORT_SYMBOL(up);
-EXPORT_SYMBOL(down_read);
-EXPORT_SYMBOL(down_write);
-EXPORT_SYMBOL(up_read);
-EXPORT_SYMBOL(up_write);
+#endif

/*
* SMP-specific symbols.
--- 2.4.4/arch/alpha/kernel/semaphore.c Wed Apr 18 04:19:24 2001
+++ linux/arch/alpha/kernel/semaphore.c Thu May 3 14:41:29 2001
@@ -201,6 +201,8 @@ __up_wakeup(struct semaphore *sem)
wake_up(&sem->wait);
}

+#if WAITQUEUE_DEBUG || DEBUG_SEMAPHORE
+
void
down(struct semaphore *sem)
{
@@ -263,3 +265,5 @@ up(struct semaphore *sem)
#endif
__up(sem);
}
+
+#endif
--- 2.4.4/arch/alpha/config.in Fri Apr 27 20:33:29 2001
+++ linux/arch/alpha/config.in Fri Apr 27 20:33:41 2001
@@ -5,8 +5,8 @@

define_bool CONFIG_ALPHA y
define_bool CONFIG_UID16 n
-define_bool CONFIG_RWSEM_GENERIC_SPINLOCK y
-define_bool CONFIG_RWSEM_XCHGADD_ALGORITHM n
+define_bool CONFIG_RWSEM_GENERIC_SPINLOCK n
+define_bool CONFIG_RWSEM_XCHGADD_ALGORITHM y

mainmenu_name "Kernel configuration of Linux for Alpha machines"

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