> 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...
okay, i agree that it's dangerous.
> ...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.
oh, we've been through bigger pain before, just to save single
instructions :)
but in this case it makes those codepaths really volatile - and
spin_lock/spin_unlock can cause an unexpected preemption. Especially on
architectures that implement smp_send_reschedule() via the SMP call
function, which in turn uses a spinlock, this approach poses real
immediate problems.
I've attached my current tree against 2.5.21, it has this optimization
removed. [i kept the spinlock.h enhancements, they might come handy in
less complex scenarios.]
Ingo
diff -rNu linux-2.5.21-final/arch/i386/kernel/entry.S linux/arch/i386/kernel/entry.S
--- linux-2.5.21-final/arch/i386/kernel/entry.S Sun Jun 9 07:28:19 2002
+++ linux/arch/i386/kernel/entry.S Fri Jun 14 00:00:16 2002
@@ -193,6 +193,7 @@
ENTRY(ret_from_fork)
#if CONFIG_SMP || CONFIG_PREEMPT
+ # NOTE: this function takes a parameter but it's unused on x86.
call schedule_tail
#endif
GET_THREAD_INFO(%ebx)
diff -rNu linux-2.5.21-final/fs/pipe.c linux/fs/pipe.c
--- linux-2.5.21-final/fs/pipe.c Sun Jun 9 07:26:29 2002
+++ linux/fs/pipe.c Thu Jun 13 23:59:56 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 -rNu linux-2.5.21-final/include/asm-i386/system.h linux/include/asm-i386/system.h
--- linux-2.5.21-final/include/asm-i386/system.h Sun Jun 9 07:26:29 2002
+++ linux/include/asm-i386/system.h Fri Jun 14 00:00:16 2002
@@ -11,9 +11,12 @@
struct task_struct; /* one of the stranger aspects of C forward declarations.. */
extern void FASTCALL(__switch_to(struct task_struct *prev, struct task_struct *next));
-#define prepare_to_switch() do { } while(0)
+#define prepare_arch_schedule(prev) do { } while(0)
+#define finish_arch_schedule(prev) do { } while(0)
+#define prepare_arch_switch(rq) do { } while(0)
+#define finish_arch_switch(rq) spin_unlock_irq(&(rq)->lock)
-#define switch_to(prev,next) do { \
+#define switch_to(prev,next,last) do { \
asm volatile("pushl %%esi\n\t" \
"pushl %%edi\n\t" \
"pushl %%ebp\n\t" \
diff -rNu linux-2.5.21-final/include/linux/sched.h linux/include/linux/sched.h
--- linux-2.5.21-final/include/linux/sched.h Sun Jun 9 07:27:21 2002
+++ linux/include/linux/sched.h Thu Jun 13 23:59:56 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 -rNu linux-2.5.21-final/include/linux/spinlock.h linux/include/linux/spinlock.h
--- linux-2.5.21-final/include/linux/spinlock.h Sun Jun 9 07:30:01 2002
+++ linux/include/linux/spinlock.h Fri Jun 14 00:00:09 2002
@@ -26,6 +26,7 @@
#define write_lock_bh(lock) do { local_bh_disable(); write_lock(lock); } while (0)
#define spin_unlock_irqrestore(lock, flags) do { spin_unlock(lock); local_irq_restore(flags); } while (0)
+#define _raw_spin_unlock_irqrestore(lock, flags) do { _raw_spin_unlock(lock); local_irq_restore(flags); } while (0)
#define spin_unlock_irq(lock) do { spin_unlock(lock); local_irq_enable(); } while (0)
#define spin_unlock_bh(lock) do { spin_unlock(lock); local_bh_enable(); } while (0)
@@ -183,6 +184,12 @@
preempt_schedule(); \
} while (0)
+#define preempt_check_resched() \
+do { \
+ if (unlikely(test_thread_flag(TIF_NEED_RESCHED))) \
+ preempt_schedule(); \
+} while (0)
+
#define spin_lock(lock) \
do { \
preempt_disable(); \
@@ -197,6 +204,12 @@
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 +219,22 @@
#else
-#define preempt_get_count() (0)
-#define preempt_disable() do { } while (0)
+#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 preempt_enable() do { } while (0)
+#define preempt_check_resched() 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 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 -rNu linux-2.5.21-final/kernel/ksyms.c linux/kernel/ksyms.c
--- linux-2.5.21-final/kernel/ksyms.c Sun Jun 9 07:26:33 2002
+++ linux/kernel/ksyms.c Thu Jun 13 23:59:56 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 -rNu linux-2.5.21-final/kernel/sched.c linux/kernel/sched.c
--- linux-2.5.21-final/kernel/sched.c Sun Jun 9 07:28:13 2002
+++ linux/kernel/sched.c Fri Jun 14 00:00:21 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);
}
/*
@@ -401,19 +435,18 @@
}
#if CONFIG_SMP || CONFIG_PREEMPT
-asmlinkage void schedule_tail(void)
+asmlinkage void schedule_tail(task_t *prev)
{
- spin_unlock_irq(&this_rq()->frozen);
+ finish_arch_switch(this_rq());
+ finish_arch_schedule(prev);
}
#endif
-static inline void context_switch(task_t *prev, task_t *next)
+static inline task_t * context_switch(task_t *prev, task_t *next)
{
struct mm_struct *mm = next->mm;
struct mm_struct *oldmm = prev->active_mm;
- prepare_to_switch();
-
if (unlikely(!mm)) {
next->active_mm = oldmm;
atomic_inc(&oldmm->mm_count);
@@ -427,7 +460,9 @@
}
/* Here we just switch the register state and the stack. */
- switch_to(prev, next);
+ switch_to(prev, next, prev);
+
+ return prev;
}
unsigned long nr_running(void)
@@ -773,6 +808,7 @@
rq = this_rq();
release_kernel_lock(prev, smp_processor_id());
+ prepare_arch_schedule(prev);
prev->sleep_timestamp = jiffies;
spin_lock_irq(&rq->lock);
@@ -828,28 +864,20 @@
if (likely(prev != next)) {
rq->nr_switches++;
rq->curr = next;
- spin_lock(&rq->frozen);
- spin_unlock(&rq->lock);
-
- context_switch(prev, next);
-
- /*
- * The runqueue pointer might be from another CPU
- * if the new task was last running on a different
- * CPU - thus re-load it.
- */
- mb();
+
+ prepare_arch_switch(rq);
+ prev = context_switch(prev, next);
+ barrier();
rq = this_rq();
- spin_unlock_irq(&rq->frozen);
- } else {
+ finish_arch_switch(rq);
+ } else
spin_unlock_irq(&rq->lock);
- }
+ finish_arch_schedule(prev);
reacquire_kernel_lock(current);
preempt_enable_no_resched();
if (test_thread_flag(TIF_NEED_RESCHED))
goto need_resched;
- return;
}
#ifdef CONFIG_PREEMPT
@@ -880,7 +908,7 @@
* 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 +919,7 @@
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 +933,36 @@
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 +1389,7 @@
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 +1399,6 @@
* 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 +1415,7 @@
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 +1643,6 @@
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 +1730,15 @@
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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