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SubjectRe: [patch] "interactivity changes", sched-2.5.64-B2

the attached patch (against your tree) fixes the SMP locking bug. The
patch also includes some other stuff i already added to my tree by that
time:

- only update the priority and do a requeueing if the sleep average has
changed. (this does not happen for pure CPU hogs or pure interactive
tasks, so no need to requeue/recalc-prio in that case.) [All the
necessary values are available at that point already, so gcc should
have an easy job making this branch really cheap.]

- do not do a full task activation in the migration-thread path - that is
supposed to be near-atomic anyway.

- fix up comments

i solved the SMP locking bug by moving the requeueing outside of
try_to_wake_up(). It does not matter that the priority update is not
atomically done now, since the current process wont do anything inbetween.
(well, it could get preempted in a preemptible kernel, but even that wont
do any harm.)

did a quick testcompile & testboot on an SMP box, appears to work.

Ingo

--- linux/kernel/sched.c.orig
+++ linux/kernel/sched.c
@@ -321,10 +321,7 @@ static int effective_prio(task_t *p)
}

/*
- * activate_task - move a task to the runqueue.
-
- * Also update all the scheduling statistics stuff. (sleep average
- * calculation, priority modifiers, etc.)
+ * __activate_task - move a task to the runqueue.
*/
static inline void __activate_task(task_t *p, runqueue_t *rq)
{
@@ -332,9 +329,16 @@ static inline void __activate_task(task_
nr_running_inc(rq);
}

-static inline void activate_task(task_t *p, runqueue_t *rq)
+/*
+ * activate_task - move a task to the runqueue and do priority recalculation
+ *
+ * Update all the scheduling statistics stuff. (sleep average
+ * calculation, priority modifiers, etc.)
+ */
+static inline int activate_task(task_t *p, runqueue_t *rq)
{
unsigned long sleep_time = jiffies - p->last_run;
+ int requeue_waker = 0;

if (sleep_time) {
int sleep_avg;
@@ -357,23 +361,25 @@ static inline void activate_task(task_t
*/
if (sleep_avg > MAX_SLEEP_AVG) {
if (!in_interrupt()) {
- prio_array_t *array = current->array;
- BUG_ON(!array);
sleep_avg += current->sleep_avg - MAX_SLEEP_AVG;
if (sleep_avg > MAX_SLEEP_AVG)
sleep_avg = MAX_SLEEP_AVG;

- current->sleep_avg = sleep_avg;
- dequeue_task(current, array);
- current->prio = effective_prio(current);
- enqueue_task(current, array);
+ if (current->sleep_avg != sleep_avg) {
+ current->sleep_avg = sleep_avg;
+ requeue_waker = 1;
+ }
}
sleep_avg = MAX_SLEEP_AVG;
}
- p->sleep_avg = sleep_avg;
- p->prio = effective_prio(p);
+ if (p->sleep_avg != sleep_avg) {
+ p->sleep_avg = sleep_avg;
+ p->prio = effective_prio(p);
+ }
}
__activate_task(p, rq);
+
+ return requeue_waker;
}

/*
@@ -486,8 +492,8 @@ void kick_if_running(task_t * p)
*/
static int try_to_wake_up(task_t * p, unsigned int state, int sync)
{
+ int success = 0, requeue_waker = 0;
unsigned long flags;
- int success = 0;
long old_state;
runqueue_t *rq;

@@ -513,7 +519,7 @@ repeat_lock_task:
if (sync)
__activate_task(p, rq);
else {
- activate_task(p, rq);
+ requeue_waker = activate_task(p, rq);
if (p->prio < rq->curr->prio)
resched_task(rq->curr);
}
@@ -523,6 +529,21 @@ repeat_lock_task:
}
task_rq_unlock(rq, &flags);

+ /*
+ * We have to do this outside the other spinlock, the two
+ * runqueues might be different:
+ */
+ if (requeue_waker) {
+ prio_array_t *array;
+
+ rq = task_rq_lock(current, &flags);
+ array = current->array;
+ dequeue_task(current, array);
+ current->prio = effective_prio(current);
+ enqueue_task(current, array);
+ task_rq_unlock(rq, &flags);
+ }
+
return success;
}

@@ -2360,7 +2381,7 @@ repeat:
set_task_cpu(p, cpu_dest);
if (p->array) {
deactivate_task(p, rq_src);
- activate_task(p, rq_dest);
+ __activate_task(p, rq_dest);
if (p->prio < rq_dest->curr->prio)
resched_task(rq_dest->curr);
}
-
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