[RFC v2 4/7] Fix the update of the total -deadline utilization

Luca Abeni
2016-04-01T15:19:34Z

Now that the inactive timer can be armed to fire at the 0-lag time,
it is possible to use inactive_task_timer() to update the total
-deadline utilization (dl_b->total_bw) at the correct time, fixing
dl_overflow() and __setparam_dl().

Signed-off-by: Luca Abeni <luca.abeni@unitn.it>
---
kernel/sched/core.c | 36 ++++++++++++------------------------
kernel/sched/deadline.c | 33 +++++++++++++++++++++++++--------
2 files changed, 37 insertions(+), 32 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 23d235c..4158d1f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2347,9 +2347,6 @@ static inline int dl_bw_cpus(int i)
* allocated bandwidth to reflect the new situation.
*
* This function is called while holding p's rq->lock.
- *
- * XXX we should delay bw change until the task's 0-lag point, see
- * __setparam_dl().
*/
static int dl_overflow(struct task_struct *p, int policy,
const struct sched_attr *attr)
@@ -2377,11 +2374,22 @@ static int dl_overflow(struct task_struct *p, int policy,
err = 0;
} else if (dl_policy(policy) && task_has_dl_policy(p) &&
!__dl_overflow(dl_b, cpus, p->dl.dl_bw, new_bw)) {
+ /*
+ * XXX this is slightly incorrect: when the task
+ * utilization decreases, we should delay the total
+ * utilization change until the task's 0-lag point.
+ * But this would require to set the task's "inactive
+ * timer" when the task is not inactive.
+ */
__dl_clear(dl_b, p->dl.dl_bw);
__dl_add(dl_b, new_bw);
err = 0;
} else if (!dl_policy(policy) && task_has_dl_policy(p)) {
- __dl_clear(dl_b, p->dl.dl_bw);
+ /*
+ * Do not decrease the total deadline utilization here,
+ * switched_from_dl() will take care to do it at the correct
+ * (0-lag) time.
+ */
err = 0;
}
raw_spin_unlock(&dl_b->lock);
@@ -3647,26 +3655,6 @@ __setparam_dl(struct task_struct *p, const struct sched_attr *attr)
dl_se->dl_period = attr->sched_period ?: dl_se->dl_deadline;
dl_se->flags = attr->sched_flags;
dl_se->dl_bw = to_ratio(dl_se->dl_period, dl_se->dl_runtime);
-
- /*
- * Changing the parameters of a task is 'tricky' and we're not doing
- * the correct thing -- also see task_dead_dl() and switched_from_dl().
- *
- * What we SHOULD do is delay the bandwidth release until the 0-lag
- * point. This would include retaining the task_struct until that time
- * and change dl_overflow() to not immediately decrement the current
- * amount.
- *
- * Instead we retain the current runtime/deadline and let the new
- * parameters take effect after the current reservation period lapses.
- * This is safe (albeit pessimistic) because the 0-lag point is always
- * before the current scheduling deadline.
- *
- * We can still have temporary overloads because we do not delay the
- * change in bandwidth until that time; so admission control is
- * not on the safe side. It does however guarantee tasks will never
- * consume more than promised.
- */
}

/*
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 97cd5f2..ca7910a 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -98,8 +98,14 @@ static void task_go_inactive(struct task_struct *p)
*/
if (ktime_us_delta(act, now) < 0) {
sub_running_bw(dl_se, dl_rq);
- if (!dl_task(p))
+ if (!dl_task(p)) {
+ struct dl_bw *dl_b = dl_bw_of(task_cpu(p));
+
+ raw_spin_lock(&dl_b->lock);
+ __dl_clear(dl_b, p->dl.dl_bw);
__dl_clear_params(p);
+ raw_spin_unlock(&dl_b->lock);
+ }

return;
}
@@ -865,8 +871,13 @@ static enum hrtimer_restart inactive_task_timer(struct hrtimer *timer)

rq = task_rq_lock(p, &flags);

- if (!dl_task(p)) {
+ if (!dl_task(p) || p->state == TASK_DEAD) {
+ struct dl_bw *dl_b = dl_bw_of(task_cpu(p));
+
+ raw_spin_lock(&dl_b->lock);
+ __dl_clear(dl_b, p->dl.dl_bw);
__dl_clear_params(p);
+ raw_spin_unlock(&dl_b->lock);

goto unlock;
}
@@ -1341,15 +1352,21 @@ static void task_fork_dl(struct task_struct *p)

static void task_dead_dl(struct task_struct *p)
{
- struct dl_bw *dl_b = dl_bw_of(task_cpu(p));
-
/*
* Since we are TASK_DEAD we won't slip out of the domain!
*/
- raw_spin_lock_irq(&dl_b->lock);
- /* XXX we should retain the bw until 0-lag */
- dl_b->total_bw -= p->dl.dl_bw;
- raw_spin_unlock_irq(&dl_b->lock);
+ if (!hrtimer_active(&p->dl.inactive_timer)) {
+ struct dl_bw *dl_b = dl_bw_of(task_cpu(p));
+
+ /*
+ * If the "inactive timer is not active, the 0-lag time
+ * is already passed, so we immediately decrease the
+ * total deadline utilization
+ */
+ raw_spin_lock_irq(&dl_b->lock);
+ __dl_clear(dl_b, p->dl.dl_bw);
+ raw_spin_unlock_irq(&dl_b->lock);
+ }
}

static void set_curr_task_dl(struct rq *rq)
--
2.5.0