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Subject[patch] semaphore optimisation

This patch modestly improves the scalability and straight-line
performance of x86 semaphores by removing the semaphore_lock and using
the per-semaphore lock instead. If removes several spinlock operations
and allows concurrent operations on separate semaphores.

No bugs were harmed in the preparation of this patch. It's just me
fartarsing aound.




--- linux-2.4.0-test11-pre7/include/linux/sched.h Sat Nov 18 13:55:32 2000
+++ linux-akpm/include/linux/sched.h Sat Nov 18 14:42:26 2000
@@ -535,6 +535,7 @@
#define CURRENT_TIME (xtime.tv_sec)

extern void FASTCALL(__wake_up(wait_queue_head_t *q, unsigned int mode, unsigned int wq_mode));
+extern void FASTCALL(____wake_up(wait_queue_head_t *q, unsigned int mode, unsigned int wq_mode));
extern void FASTCALL(__wake_up_sync(wait_queue_head_t *q, unsigned int mode, unsigned int wq_mode));
extern void FASTCALL(sleep_on(wait_queue_head_t *q));
extern long FASTCALL(sleep_on_timeout(wait_queue_head_t *q,
--- linux-2.4.0-test11-pre7/include/linux/wait.h Sat Nov 18 13:55:32 2000
+++ linux-akpm/include/linux/wait.h Sat Nov 18 14:42:26 2000
@@ -72,6 +72,7 @@
# define wq_read_unlock_irqrestore read_unlock_irqrestore
# define wq_read_unlock read_unlock
# define wq_write_lock_irq write_lock_irq
+# define wq_write_unlock_irq write_unlock_irq
# define wq_write_lock_irqsave write_lock_irqsave
# define wq_write_unlock_irqrestore write_unlock_irqrestore
# define wq_write_unlock write_unlock
@@ -84,6 +85,7 @@
# define wq_read_unlock spin_unlock
# define wq_read_unlock_irqrestore spin_unlock_irqrestore
# define wq_write_lock_irq spin_lock_irq
+# define wq_write_unlock_irq spin_unlock_irq
# define wq_write_lock_irqsave spin_lock_irqsave
# define wq_write_unlock_irqrestore spin_unlock_irqrestore
# define wq_write_unlock spin_unlock
--- linux-2.4.0-test11-pre7/arch/i386/kernel/semaphore.c Sat Nov 18 13:55:28 2000
+++ linux-akpm/arch/i386/kernel/semaphore.c Sat Nov 18 14:42:26 2000
@@ -53,16 +53,15 @@
wake_up(&sem->wait);
}

-static spinlock_t semaphore_lock = SPIN_LOCK_UNLOCKED;
-
void __down(struct semaphore * sem)
{
struct task_struct *tsk = current;
DECLARE_WAITQUEUE(wait, tsk);
- tsk->state = TASK_UNINTERRUPTIBLE;
- add_wait_queue_exclusive(&sem->wait, &wait);

- spin_lock_irq(&semaphore_lock);
+ tsk->state = TASK_UNINTERRUPTIBLE;
+ wq_write_lock_irq(&sem->wait.lock);
+ wait.flags = WQ_FLAG_EXCLUSIVE;
+ __add_wait_queue_tail(&sem->wait, &wait);
sem->sleepers++;
for (;;) {
int sleepers = sem->sleepers;
@@ -76,14 +75,14 @@
break;
}
sem->sleepers = 1; /* us - see -1 above */
- spin_unlock_irq(&semaphore_lock);
+ wq_write_unlock_irq(&sem->wait.lock);

schedule();
tsk->state = TASK_UNINTERRUPTIBLE;
- spin_lock_irq(&semaphore_lock);
+ wq_write_lock_irq(&sem->wait.lock);
}
- spin_unlock_irq(&semaphore_lock);
- remove_wait_queue(&sem->wait, &wait);
+ __remove_wait_queue(&sem->wait, &wait);
+ wq_write_unlock_irq(&sem->wait.lock);
tsk->state = TASK_RUNNING;
wake_up(&sem->wait);
}
@@ -93,10 +92,11 @@
int retval = 0;
struct task_struct *tsk = current;
DECLARE_WAITQUEUE(wait, tsk);
- tsk->state = TASK_INTERRUPTIBLE;
- add_wait_queue_exclusive(&sem->wait, &wait);

- spin_lock_irq(&semaphore_lock);
+ tsk->state = TASK_INTERRUPTIBLE;
+ wq_write_lock_irq(&sem->wait.lock);
+ wait.flags = WQ_FLAG_EXCLUSIVE;
+ __add_wait_queue_tail(&sem->wait, &wait);
sem->sleepers ++;
for (;;) {
int sleepers = sem->sleepers;
@@ -126,15 +126,15 @@
break;
}
sem->sleepers = 1; /* us - see -1 above */
- spin_unlock_irq(&semaphore_lock);
+ wq_write_unlock_irq(&sem->wait.lock);

schedule();
tsk->state = TASK_INTERRUPTIBLE;
- spin_lock_irq(&semaphore_lock);
+ wq_write_lock_irq(&sem->wait.lock);
}
- spin_unlock_irq(&semaphore_lock);
+ __remove_wait_queue(&sem->wait, &wait);
+ wq_write_unlock_irq(&sem->wait.lock);
tsk->state = TASK_RUNNING;
- remove_wait_queue(&sem->wait, &wait);
wake_up(&sem->wait);
return retval;
}
@@ -152,7 +152,7 @@
int sleepers;
unsigned long flags;

- spin_lock_irqsave(&semaphore_lock, flags);
+ wq_write_lock_irqsave(&sem->wait.lock, flags);
sleepers = sem->sleepers + 1;
sem->sleepers = 0;

@@ -161,9 +161,9 @@
* playing, because we own the spinlock.
*/
if (!atomic_add_negative(sleepers, &sem->count))
- wake_up(&sem->wait);
+ ____wake_up(&sem->wait, TASK_UNINTERRUPTIBLE | TASK_INTERRUPTIBLE, WQ_FLAG_EXCLUSIVE);

- spin_unlock_irqrestore(&semaphore_lock, flags);
+ wq_write_unlock_irqrestore(&sem->wait.lock, flags);
return 1;
}

--- linux-2.4.0-test11-pre7/kernel/sched.c Sat Nov 18 13:55:32 2000
+++ linux-akpm/kernel/sched.c Sat Nov 18 14:42:26 2000
@@ -700,7 +700,7 @@
}

static inline void __wake_up_common (wait_queue_head_t *q, unsigned int mode,
- unsigned int wq_mode, const int sync)
+ unsigned int wq_mode, const int sync, const int do_locking)
{
struct list_head *tmp, *head;
struct task_struct *p, *best_exclusive;
@@ -713,7 +713,8 @@
best_cpu = smp_processor_id();
irq = in_interrupt();
best_exclusive = NULL;
- wq_write_lock_irqsave(&q->lock, flags);
+ if (do_locking)
+ wq_write_lock_irqsave(&q->lock, flags);

#if WAITQUEUE_DEBUG
CHECK_MAGIC_WQHEAD(q);
@@ -768,19 +769,25 @@
else
wake_up_process(best_exclusive);
}
- wq_write_unlock_irqrestore(&q->lock, flags);
+ if (do_locking)
+ wq_write_unlock_irqrestore(&q->lock, flags);
out:
return;
}

void __wake_up(wait_queue_head_t *q, unsigned int mode, unsigned int wq_mode)
{
- __wake_up_common(q, mode, wq_mode, 0);
+ __wake_up_common(q, mode, wq_mode, 0, 1);
+}
+
+void ____wake_up(wait_queue_head_t *q, unsigned int mode, unsigned int wq_mode)
+{
+ __wake_up_common(q, mode, wq_mode, 0, 0);
}

void __wake_up_sync(wait_queue_head_t *q, unsigned int mode, unsigned int wq_mode)
{
- __wake_up_common(q, mode, wq_mode, 1);
+ __wake_up_common(q, mode, wq_mode, 1, 1);
}

#define SLEEP_ON_VAR \
-
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