> + * WARNING: to squeeze out a few more cycles we do not disable preemption
> + * explicitly (or implicitly), we just keep interrupts disabled. This means
> + * that within task_rq_lock/unlock sections you must be careful
> + * about locking/unlocking spinlocks, since they could cause an unexpected
> + * preemption.
This is not working for me. I am getting intermittent hard locks -
seems we are taking a hard fall with interrupts off. Smelt like the
preemption optimization, so I removed it and all is well...
...which is fine, because I do not entirely trust this and it is really
not worth it to save just an inc and a dec.
Attached is your #3 patch with that one change reverted. The rest of
the changes are in tact. It is running here very nicely...
Unless you have a solution - and really think this is worth it - I would
like to pull this change out.
Robert Love
diff -urN linux-2.5.21/fs/pipe.c linux/fs/pipe.c
--- linux-2.5.21/fs/pipe.c Sat Jun 8 22:26:29 2002
+++ linux/fs/pipe.c Thu Jun 13 14:10:48 2002
@@ -119,7 +119,7 @@
* writers synchronously that there is more
* room.
*/
- wake_up_interruptible(PIPE_WAIT(*inode));
+ wake_up_interruptible_sync(PIPE_WAIT(*inode));
kill_fasync(PIPE_FASYNC_WRITERS(*inode), SIGIO, POLL_OUT);
if (!PIPE_EMPTY(*inode))
BUG();
@@ -219,7 +219,7 @@
* is going to give up this CPU, so it doesnt have
* to do idle reschedules.
*/
- wake_up_interruptible(PIPE_WAIT(*inode));
+ wake_up_interruptible_sync(PIPE_WAIT(*inode));
kill_fasync(PIPE_FASYNC_READERS(*inode), SIGIO, POLL_IN);
PIPE_WAITING_WRITERS(*inode)++;
pipe_wait(inode);
diff -urN linux-2.5.21/include/linux/sched.h linux/include/linux/sched.h
--- linux-2.5.21/include/linux/sched.h Sat Jun 8 22:27:21 2002
+++ linux/include/linux/sched.h Thu Jun 13 14:10:48 2002
@@ -491,6 +491,7 @@
extern unsigned long prof_shift;
extern void FASTCALL(__wake_up(wait_queue_head_t *q, unsigned int mode, int nr));
+extern void FASTCALL(__wake_up_sync(wait_queue_head_t *q, unsigned int mode, int nr));
extern void FASTCALL(sleep_on(wait_queue_head_t *q));
extern long FASTCALL(sleep_on_timeout(wait_queue_head_t *q,
signed long timeout));
@@ -507,6 +508,11 @@
#define wake_up_interruptible(x) __wake_up((x),TASK_INTERRUPTIBLE, 1)
#define wake_up_interruptible_nr(x, nr) __wake_up((x),TASK_INTERRUPTIBLE, nr)
#define wake_up_interruptible_all(x) __wake_up((x),TASK_INTERRUPTIBLE, 0)
+#ifdef CONFIG_SMP
+#define wake_up_interruptible_sync(x) __wake_up_sync((x),TASK_INTERRUPTIBLE, 1)
+#else
+#define wake_up_interruptible_sync(x) __wake_up((x),TASK_INTERRUPTIBLE, 1)
+#endif
asmlinkage long sys_wait4(pid_t pid,unsigned int * stat_addr, int options, struct rusage * ru);
extern int in_group_p(gid_t);
diff -urN linux-2.5.21/include/linux/spinlock.h linux/include/linux/spinlock.h
--- linux-2.5.21/include/linux/spinlock.h Sat Jun 8 22:30:01 2002
+++ linux/include/linux/spinlock.h Thu Jun 13 14:11:43 2002
@@ -197,6 +197,12 @@
preempt_enable(); \
} while (0)
+#define spin_unlock_no_resched(lock) \
+do { \
+ _raw_spin_unlock(lock); \
+ preempt_enable_no_resched(); \
+} while (0)
+
#define read_lock(lock) ({preempt_disable(); _raw_read_lock(lock);})
#define read_unlock(lock) ({_raw_read_unlock(lock); preempt_enable();})
#define write_lock(lock) ({preempt_disable(); _raw_write_lock(lock);})
@@ -206,20 +212,21 @@
#else
-#define preempt_get_count() (0)
-#define preempt_disable() do { } while (0)
-#define preempt_enable_no_resched() do {} while(0)
-#define preempt_enable() do { } while (0)
-
-#define spin_lock(lock) _raw_spin_lock(lock)
-#define spin_trylock(lock) _raw_spin_trylock(lock)
-#define spin_unlock(lock) _raw_spin_unlock(lock)
-
-#define read_lock(lock) _raw_read_lock(lock)
-#define read_unlock(lock) _raw_read_unlock(lock)
-#define write_lock(lock) _raw_write_lock(lock)
-#define write_unlock(lock) _raw_write_unlock(lock)
-#define write_trylock(lock) _raw_write_trylock(lock)
+#define preempt_get_count() (0)
+#define preempt_disable() do {} while (0)
+#define preempt_enable_no_resched() do {} while (0)
+#define preempt_enable() do {} while (0)
+
+#define spin_lock(lock) _raw_spin_lock(lock)
+#define spin_trylock(lock) _raw_spin_trylock(lock)
+#define spin_unlock(lock) _raw_spin_unlock(lock)
+#define spin_unlock_no_resched(lock) _raw_spin_unlock(lock)
+
+#define read_lock(lock) _raw_read_lock(lock)
+#define read_unlock(lock) _raw_read_unlock(lock)
+#define write_lock(lock) _raw_write_lock(lock)
+#define write_unlock(lock) _raw_write_unlock(lock)
+#define write_trylock(lock) _raw_write_trylock(lock)
#endif
/* "lock on reference count zero" */
diff -urN linux-2.5.21/kernel/ksyms.c linux/kernel/ksyms.c
--- linux-2.5.21/kernel/ksyms.c Sat Jun 8 22:26:33 2002
+++ linux/kernel/ksyms.c Thu Jun 13 14:10:48 2002
@@ -457,6 +457,9 @@
/* process management */
EXPORT_SYMBOL(complete_and_exit);
EXPORT_SYMBOL(__wake_up);
+#if CONFIG_SMP
+EXPORT_SYMBOL_GPL(__wake_up_sync); /* internal use only */
+#endif
EXPORT_SYMBOL(wake_up_process);
EXPORT_SYMBOL(sleep_on);
EXPORT_SYMBOL(sleep_on_timeout);
diff -urN linux-2.5.21/kernel/sched.c linux/kernel/sched.c
--- linux-2.5.21/kernel/sched.c Sat Jun 8 22:28:13 2002
+++ linux/kernel/sched.c Thu Jun 13 14:10:48 2002
@@ -135,7 +135,6 @@
*/
struct runqueue {
spinlock_t lock;
- spinlock_t frozen;
unsigned long nr_running, nr_switches, expired_timestamp;
signed long nr_uninterruptible;
task_t *curr, *idle;
@@ -153,17 +152,21 @@
#define cpu_curr(cpu) (cpu_rq(cpu)->curr)
#define rt_task(p) ((p)->prio < MAX_RT_PRIO)
+/*
+ * task_rq_lock - lock the runqueue a given task resides on and disable
+ * interrupts. Note the ordering: we can safely lookup the task_rq without
+ * explicitly disabling preemption.
+ */
static inline runqueue_t *task_rq_lock(task_t *p, unsigned long *flags)
{
struct runqueue *rq;
repeat_lock_task:
- preempt_disable();
+ local_irq_save(*flags);
rq = task_rq(p);
- spin_lock_irqsave(&rq->lock, *flags);
+ spin_lock(&rq->lock);
if (unlikely(rq != task_rq(p))) {
spin_unlock_irqrestore(&rq->lock, *flags);
- preempt_enable();
goto repeat_lock_task;
}
return rq;
@@ -172,7 +175,23 @@
static inline void task_rq_unlock(runqueue_t *rq, unsigned long *flags)
{
spin_unlock_irqrestore(&rq->lock, *flags);
- preempt_enable();
+}
+
+/*
+ * rq_lock - lock a given runqueue and disable interrupts.
+ */
+static inline runqueue_t *rq_lock(runqueue_t *rq)
+{
+ local_irq_disable();
+ rq = this_rq();
+ spin_lock(&rq->lock);
+ return rq;
+}
+
+static inline void rq_unlock(runqueue_t *rq)
+{
+ spin_unlock(&rq->lock);
+ local_irq_enable();
}
/*
@@ -284,9 +303,15 @@
repeat:
preempt_disable();
rq = task_rq(p);
- while (unlikely(rq->curr == p)) {
+ if (unlikely(rq->curr == p)) {
cpu_relax();
- barrier();
+ /*
+ * enable/disable preemption just to make this
+ * a preemption point - we are busy-waiting
+ * anyway.
+ */
+ preempt_enable();
+ goto repeat;
}
rq = task_rq_lock(p, &flags);
if (unlikely(rq->curr == p)) {
@@ -322,40 +347,50 @@
* "current->state = TASK_RUNNING" to mark yourself runnable
* without the overhead of this.
*/
-static int try_to_wake_up(task_t * p)
+static int try_to_wake_up(task_t * p, int sync)
{
unsigned long flags;
int success = 0;
long old_state;
runqueue_t *rq;
+repeat_lock_task:
rq = task_rq_lock(p, &flags);
old_state = p->state;
- p->state = TASK_RUNNING;
if (!p->array) {
+ if (unlikely(sync && (rq->curr != p))) {
+ if (p->thread_info->cpu != smp_processor_id()) {
+ p->thread_info->cpu = smp_processor_id();
+ task_rq_unlock(rq, &flags);
+ goto repeat_lock_task;
+ }
+ }
if (old_state == TASK_UNINTERRUPTIBLE)
rq->nr_uninterruptible--;
activate_task(p, rq);
+ /*
+ * If sync is set, a resched_task() is a NOOP
+ */
if (p->prio < rq->curr->prio)
resched_task(rq->curr);
success = 1;
}
+ p->state = TASK_RUNNING;
task_rq_unlock(rq, &flags);
+
return success;
}
int wake_up_process(task_t * p)
{
- return try_to_wake_up(p);
+ return try_to_wake_up(p, 0);
}
void wake_up_forked_process(task_t * p)
{
runqueue_t *rq;
- preempt_disable();
- rq = this_rq();
- spin_lock_irq(&rq->lock);
+ rq = rq_lock(rq);
p->state = TASK_RUNNING;
if (!rt_task(p)) {
@@ -371,8 +406,7 @@
p->thread_info->cpu = smp_processor_id();
activate_task(p, rq);
- spin_unlock_irq(&rq->lock);
- preempt_enable();
+ rq_unlock(rq);
}
/*
@@ -403,7 +437,7 @@
#if CONFIG_SMP || CONFIG_PREEMPT
asmlinkage void schedule_tail(void)
{
- spin_unlock_irq(&this_rq()->frozen);
+ spin_unlock_irq(&this_rq()->lock);
}
#endif
@@ -828,9 +862,6 @@
if (likely(prev != next)) {
rq->nr_switches++;
rq->curr = next;
- spin_lock(&rq->frozen);
- spin_unlock(&rq->lock);
-
context_switch(prev, next);
/*
@@ -840,10 +871,8 @@
*/
mb();
rq = this_rq();
- spin_unlock_irq(&rq->frozen);
- } else {
- spin_unlock_irq(&rq->lock);
}
+ spin_unlock_irq(&rq->lock);
reacquire_kernel_lock(current);
preempt_enable_no_resched();
@@ -880,7 +909,7 @@
* started to run but is not in state TASK_RUNNING. try_to_wake_up() returns
* zero in this (rare) case, and we handle it by continuing to scan the queue.
*/
-static inline void __wake_up_common(wait_queue_head_t *q, unsigned int mode, int nr_exclusive)
+static inline void __wake_up_common(wait_queue_head_t *q, unsigned int mode, int nr_exclusive, int sync)
{
struct list_head *tmp;
unsigned int state;
@@ -891,7 +920,7 @@
curr = list_entry(tmp, wait_queue_t, task_list);
p = curr->task;
state = p->state;
- if ((state & mode) && try_to_wake_up(p) &&
+ if ((state & mode) && try_to_wake_up(p, sync) &&
((curr->flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive))
break;
}
@@ -905,17 +934,36 @@
return;
spin_lock_irqsave(&q->lock, flags);
- __wake_up_common(q, mode, nr_exclusive);
+ __wake_up_common(q, mode, nr_exclusive, 0);
+ spin_unlock_irqrestore(&q->lock, flags);
+}
+
+#if CONFIG_SMP
+
+void __wake_up_sync(wait_queue_head_t *q, unsigned int mode, int nr_exclusive)
+{
+ unsigned long flags;
+
+ if (unlikely(!q))
+ return;
+
+ spin_lock_irqsave(&q->lock, flags);
+ if (likely(nr_exclusive))
+ __wake_up_common(q, mode, nr_exclusive, 1);
+ else
+ __wake_up_common(q, mode, nr_exclusive, 0);
spin_unlock_irqrestore(&q->lock, flags);
}
+#endif
+
void complete(struct completion *x)
{
unsigned long flags;
spin_lock_irqsave(&x->wait.lock, flags);
x->done++;
- __wake_up_common(&x->wait, TASK_UNINTERRUPTIBLE | TASK_INTERRUPTIBLE, 1);
+ __wake_up_common(&x->wait, TASK_UNINTERRUPTIBLE | TASK_INTERRUPTIBLE, 1, 0);
spin_unlock_irqrestore(&x->wait.lock, flags);
}
@@ -1342,8 +1390,7 @@
runqueue_t *rq;
prio_array_t *array;
- preempt_disable();
- rq = this_rq();
+ rq = rq_lock(rq);
/*
* Decrease the yielding task's priority by one, to avoid
@@ -1353,7 +1400,6 @@
* If priority is already MAX_PRIO-1 then we still
* roundrobin the task within the runlist.
*/
- spin_lock_irq(&rq->lock);
array = current->array;
/*
* If the task has reached maximum priority (or is a RT task)
@@ -1370,8 +1416,7 @@
list_add_tail(¤t->run_list, array->queue + current->prio);
__set_bit(current->prio, array->bitmap);
}
- spin_unlock(&rq->lock);
- preempt_enable_no_resched();
+ spin_unlock_no_resched(&rq->lock);
schedule();
@@ -1599,7 +1644,6 @@
rq->active = rq->arrays;
rq->expired = rq->arrays + 1;
spin_lock_init(&rq->lock);
- spin_lock_init(&rq->frozen);
INIT_LIST_HEAD(&rq->migration_queue);
for (j = 0; j < 2; j++) {
@@ -1687,7 +1731,15 @@
task_rq_unlock(rq, &flags);
goto out;
}
-
+ /*
+ * If the task is not on a runqueue (and not running), then
+ * it is sufficient to simply update the task's cpu field.
+ */
+ if (!p->array && (p != rq->curr)) {
+ p->thread_info->cpu = __ffs(p->cpus_allowed);
+ task_rq_unlock(rq, &flags);
+ goto out;
+ }
init_MUTEX_LOCKED(&req.sem);
req.task = p;
list_add(&req.list, &rq->migration_queue);
-
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