Re: Another SCHED_DEADLINE bug (with bisection and possible fix)

[Peter Zijlstra]

On Thu, Jan 15, 2015 at 02:35:46PM +0100, Luca Abeni wrote:

> >>From what I understand we should either modify the tasks run/sleep stats
> >when we change its parameters or we should schedule a delayed release of
> >the bandwidth delta (when it reaches its 0-lag point, if thats in the
> >future).

> I suspect the correct behaviour can be difficult to implement:
> - When a SCHED_DEADLINE task ends (or changes its scheduling policy to
>   something different), its bandwidth cannot be released immediately,
>   but should be released at the "0-lag time" (which reminds me about the
>   GRUB patches... I had to implement a similar behaviour in those patches :)
> - The same applies when the task changes its scheduling parameters decreasing
>   its bandwidth. In this case, we also need to update the current runtime (if
>   it is larger than the new runtime, set it to the new maximum value - I think
>   this is the safest thing to do)
> - When a task changes its parameters to increase its bandwidth, be do not
>   have such problems.
> 
> As far as I can see, if we apply the runtime / deadline changes starting from
> the next reservation period we are safe (because the "0-lag time" is always
> smaller than the current scheduling deadline).
> This might cause some transient overloads (because if I change the parameters
> of a task at time t, the update takes action a little bit later - at the next
> scheduling deadline), but guarantees that a task never consumes more than
> expected (for example: if a task continuously changes its bandwidth between
> 0.4 and 0.3, it will never consume more than 0.4. I suspect that if we
> immediately update dl_se->deadline and dl_se->runtime a task can arrive to
> consume much more CPU time).


OK, how about something like this; it seems to survive your Bug-Test for
at least 50 cycles.

---
 kernel/sched/core.c     | 33 ++++++++++++++++++++++++++++-----
 kernel/sched/deadline.c |  3 ++-
 2 files changed, 30 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ade2958a9197..d787d6553d72 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1816,6 +1816,10 @@ void __dl_clear_params(struct task_struct *p)
 	dl_se->dl_period = 0;
 	dl_se->flags = 0;
 	dl_se->dl_bw = 0;
+
+	dl_se->dl_throttled = 0;
+	dl_se->dl_new = 1;
+	dl_se->dl_yielded = 0;
 }
 
 /*
@@ -1844,7 +1848,7 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
 #endif
 
 	RB_CLEAR_NODE(&p->dl.rb_node);
-	hrtimer_init(&p->dl.dl_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	init_dl_task_timer(&p->dl);
 	__dl_clear_params(p);
 
 	INIT_LIST_HEAD(&p->rt.run_list);
@@ -2054,6 +2058,9 @@ 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)
@@ -3258,15 +3265,31 @@ __setparam_dl(struct task_struct *p, const struct sched_attr *attr)
 {
 	struct sched_dl_entity *dl_se = &p->dl;
 
-	init_dl_task_timer(dl_se);
 	dl_se->dl_runtime = attr->sched_runtime;
 	dl_se->dl_deadline = attr->sched_deadline;
 	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);
-	dl_se->dl_throttled = 0;
-	dl_se->dl_new = 1;
-	dl_se->dl_yielded = 0;
+
+	/*
+	 * 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 b52092f2636d..726470d47f87 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1094,6 +1094,7 @@ static void task_dead_dl(struct task_struct *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);
 
@@ -1614,8 +1615,8 @@ static void cancel_dl_timer(struct rq *rq, struct task_struct *p)
 
 static void switched_from_dl(struct rq *rq, struct task_struct *p)
 {
+	/* XXX we should retain the bw until 0-lag */
 	cancel_dl_timer(rq, p);
-
 	__dl_clear_params(p);
 
 	/*
--
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