On 01/29/2016 11:33 AM, Jan Stancek wrote:
>>
>> Also note that I don't think failing this test is a bug per se.
>> Undesirable maybe, but within spec, since SIGALRM is process wide, so it
>> being delivered to the SCHED_OTHER task is accepted, and SCHED_OTHER has
>> no timeliness guarantees.
>>
>> That said; if I could reliably reproduce I'd have a go at fixing this, I
>> suspect there's a 'fun' problem at the bottom of this.
> 
> Thanks for trying, I'll see if I can find some more reliable way.

I think I have found a more reliably way, however it requires an older
stable kernel: 3.12.53 up to 4.1.17.

Consider following scenario:
- all tasks on system have RT sched class
- main thread of reproducer becomes the only SCHED_OTHER task on system
- when alarm(2) expires, main thread is woken up on cpu that is occupied by
  busy looping RT thread (low_priority_thread)
- because main thread was sleeping for 2 seconds, its load has decayed to 0
- the only chance for main thread to run is if it gets balanced to idle CPU
- task_tick_fair() doesn't run, there is RT task running on this CPU
- main thread is on cfs run queue but its load stays 0
- load balancer never sees this CPU (group) as busy

Attached is reproducer and script, which tries to trigger scenario above.
I can reproduce it with 4.1.17 on baremetal 4 CPU x86_64 with about 1:50 chance.

In this setup failure state persists for a long time, perhaps indefinitely.
I tried extending RUNTIME to 10 minutes, main thread still wouldn't run.

One more clue: I could work around this issue if I forced an update_entity_load_avg()
on sched_entities that have not been updated for some time, as part of
periodic rebalance_domains() call.

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c7c1d28..1b5fe80 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5264,6 +5264,7 @@ static void update_blocked_averages(int cpu)
 	struct rq *rq = cpu_rq(cpu);
 	struct cfs_rq *cfs_rq;
 	unsigned long flags;
+	struct rb_node *rb;

 	raw_spin_lock_irqsave(&rq->lock, flags);
 	update_rq_clock(rq);
@@ -5281,6 +5282,19 @@ static void update_blocked_averages(int cpu)
 	}

 	raw_spin_unlock_irqrestore(&rq->lock, flags);
+
+	cfs_rq = &(cpu_rq(cpu)->cfs);
+	for (rb = rb_first_postorder(&cfs_rq->tasks_timeline); rb; rb = rb_next_postorder(rb)) {
+		struct sched_entity *se = rb_entry(rb, struct sched_entity, run_node);
+
+		// Task on rq has not been updated for 500ms :-(
+		if ((cfs_rq_clock_task(cfs_rq) - se->avg.last_runnable_update) > 500L * (1 << 20))
+			update_entity_load_avg(se, 1);
+	}
 }

 /*

Regards,
Jan