Fixed drift in periodic events

Before this patch, calculation for periodic events occured after the
event was dispatched, but didn't account for the overhead of the
callback itself.

Now calculation of the new target is based only on the previous target,
ignoring drift introduced by the callback. This may lead never sleeping
if the callback takes longer than its period, but the fairness of the
scheduler should avoid problems this may cause.

Additionally, equeue_enqueue was restructured to avoid calling
equeue_tick redundantly (a system call in some implementations).

equeue.c

@@ -197,10 +197,10 @@ void equeue_dealloc(equeue_t *q, void *p) {
 
 
 // equeue scheduling functions
-static int equeue_enqueue(equeue_t *q, struct equeue_event *e, unsigned ms) {
+static int equeue_enqueue(equeue_t *q, struct equeue_event *e, unsigned tick) {
     // setup event and hash local id with buffer offset for unique id
     int id = (e->id << q->npw2) | ((unsigned char *)e - q->buffer);
-    e->target = equeue_tick() + ms;
+    e->target = tick + equeue_clampdiff(e->target, tick);
     e->generation = q->generation;
 
     equeue_mutex_lock(&q->queuelock);
@@ -235,7 +235,8 @@ static int equeue_enqueue(equeue_t *q, struct equeue_event *e, unsigned ms) {
     // notify background timer
     if ((q->background.update && q->background.active) &&
         (q->queue == e && !e->sibling)) {
-        q->background.update(q->background.timer, ms);
+        q->background.update(q->background.timer,
+                equeue_clampdiff(e->target, tick));
     }
 
     equeue_mutex_unlock(&q->queuelock);
@@ -332,10 +333,11 @@ static struct equeue_event *equeue_dequeue(equeue_t *q, unsigned target) {
 
 int equeue_post(equeue_t *q, void (*cb)(void*), void *p) {
     struct equeue_event *e = (struct equeue_event*)p - 1;
+    unsigned tick = equeue_tick();
     e->cb = cb;
-    e->target = equeue_clampdiff(e->target, 0);
+    e->target = tick + e->target;
 
-    int id = equeue_enqueue(q, e, e->target);
+    int id = equeue_enqueue(q, e, tick);
     equeue_sema_signal(&q->eventsema);
     return id;
 }
@@ -380,7 +382,8 @@ void equeue_dispatch(equeue_t *q, int ms) {
 
             // reenqueue periodic events or deallocate
             if (e->period >= 0) {
-                equeue_enqueue(q, e, e->period);
+                e->target += e->period;
+                equeue_enqueue(q, e, equeue_tick());
             } else {
                 equeue_incid(q, e);
                 equeue_dealloc(q, e+1);