[PATCH] Re: current scheduler bits #3, 2.5.21

From: Robert Love (rml@tech9.net)
Date: Thu Jun 13 2002 - 16:26:34 EST


On Tue, 2002-06-11 at 11:33, Ingo Molnar wrote:

> + * 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(&current->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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This archive was generated by hypermail 2b29 : Sat Jun 15 2002 - 22:00:29 EST