Re: [PATCH] sched/deadline: Throttle a constrained task activated if overflow

[Xunlei Pang]

On 04/11/2017 at 04:47 AM, Daniel Bristot de Oliveira wrote:
> On 04/10/2017 11:22 AM, Xunlei Pang wrote:
>> I was testing Daniel's changes with his test case in the commit
>> df8eac8cafce ("sched/deadline: Throttle a constrained deadline
>> task activated after the deadline"), and tweaked it a little.
>>
>> Instead of having the runtime equal to the deadline, I tweaked
>> runtime, deadline and sleep value to ensure every time it calls
>> dl_check_constrained_dl() with "dl_se->deadline > rq_clock(rq)"
>> as well as true dl_entity_overflow(), so it does replenishing
>> every wake up in update_dl_entity(), and break its bandwidth.
>>
>> Daniel's test case had:
>> attr.sched_runtime = 2 * 1000 * 1000; /* 2 ms */
>> attr.sched_deadline = 2 * 1000 * 1000; /* 2 ms*/
>> attr.sched_period = 2 * 1000 * 1000 * 1000; /* 2 s */
>> ts.tv_sec = 0;
>> ts.tv_nsec = 2000 * 1000; /* 2 ms */
>>
>> I changed it to:
>> attr.sched_runtime = 5 * 1000 * 1000; /* 5 ms */
>> attr.sched_deadline = 7 * 1000 * 1000; /* 7 ms */
>> attr.sched_period = 1 * 1000 * 1000 * 1000; /* 1 s */
>> ts.tv_sec = 0;
>> ts.tv_nsec = 1000 * 1000; /* 1 ms */
>>
>> The change above can result in over 25% of the CPU on my machine.
>>
>> In order to avoid the beakage, we improve dl_check_constrained_dl()
>> to prevent dl tasks from being activated until the next period if it
>> runs out of bandwidth of the current period.
> The problem now is that, with your patch, we will throttle the task
> with some possible runtime. Moreover, the task did not brake any
> rule, like being awakened after the deadline - the user-space is not
> misbehaving.
>
> That is +- what the reproducer is doing when using your patch,
> (I put some trace_printk when noticing the overflow in the wakeup).
>
>           <idle>-0     [007] d.h.  1505.066439: enqueue_task_dl: my current runtime is 3657361 and the deadline is 4613027 from now 
>           <idle>-0     [007] d.h.  1505.066439: enqueue_task_dl: 	my dl_runtime is 5000000
>
> and so the task will be throttled with 3657361 ns runtime available.
>
> As we can see, it is really breaking the density:
>
> 5ms / 7ms (.714285) < 3657361 / 4613027 (.792833)

For the runtime 5ms, deadline 7ms, 3657361 is the remaining runtime, so the actual runtime is
(5000000 - 3657361) = 1342639, and the actual density is 1342639 / 7000000 (.191806),
not break the limited density 5ms / 7ms (.714285).

"3657361 / 4613027 (.792833)" means available density greater than limited density(.714285),
this means true dl_entity_overflow(), and can't meet the requirement in the current period any more,
thus need to throttle it until next period.

Regards,
Xunlei

>
> Well, it is not breaking that much. Trying to be less pessimist, we can
> compute a new runtime with following equation:
>
> runtime = (dl_runtime / dl_deadline) * (deadline - now)
>
> That is, a runtime which fits in the task's density.
>
> In code:
>
> dl_se->runtime = (div64_u64(dl_se->dl_runtime << 20, dl_se->dl_deadline) * (dl_se->deadline - rq_clock(rq))) >> 20;
>
> For example (a trace_printk) showing the adjusted runtime for the
> previous task:
>           <idle>-0     [007] d.h.  1505.066440: enqueue_task_dl: 	I can still run for 3294208 (it is not that bad)
>
> With the adjusted runtime, we have the following density:
>
> 3294208 / 4613027 = .714109
>
> as .714109  < .714285
>
> VoilÃ. We can still use 3.2 ms of runtime! Not bad at all.
>
> However, even this result is, somehow, controversial. Because we are
> reducing the task's reservation (Q/P). But that is very close to the
> implicit deadline behavior: when it "overflows", the runtime is truncated
> (a replenishment...) and the deadline is postponed. In this case, we do
> not need to postpone the deadline, just to "truncate" the runtime to fit
> in the density... it is not that bad.
>
> Another possibility is not to replenish a constrained deadline
> task twice in a period. In this case, the task would run further
> the deadline, potentially causing problems for implicit deadline tasks.
> But! even implicit deadline would do it in a overloaded system. The
> problem is that, by using the density the system easily becomes
> overloaded (too pessimistic).
>
> Resuming (so far):
>
> 1) We can be pessimistic to the constrained deadline task,
>    with Xunlei's patch;
> 2) Compute a new runtime to fit in the density - somehow pessimistic.
> 3) Avoid replenishing a constrained deadline before the next period.
>    but the system will become overload very easily (density).
>
> I think the option 2 is the mid-term.
> I am showing a _proof of concept_ patch bellow. I is based in the
> Xunlei's changes in the verification. I need to polish it... but...
> you guys can read the idea in C.
>
> I have the 3) too, but I am not sure if it is as good as 2.
>
> We still need to think more about the solution.... test more... I will
> continue working on this tomorrow, discussing with luca and tommaso
> as well.
>
> Thoughts? Am I missing something (probably, I am tired :-) )?
>
> ---
>  kernel/sched/deadline.c | 53 ++++++++++++++++++++++++++++---------------------
>  1 file changed, 30 insertions(+), 23 deletions(-)
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 7508129..6fa4887 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -696,34 +696,38 @@ void init_dl_task_timer(struct sched_dl_entity *dl_se)
>  }
>  
>  /*
> - * During the activation, CBS checks if it can reuse the current task's
> - * runtime and period. If the deadline of the task is in the past, CBS
> - * cannot use the runtime, and so it replenishes the task. This rule
> - * works fine for implicit deadline tasks (deadline == period), and the
> - * CBS was designed for implicit deadline tasks. However, a task with
> - * constrained deadline (deadine < period) might be awakened after the
> - * deadline, but before the next period. In this case, replenishing the
> - * task would allow it to run for runtime / deadline. As in this case
> - * deadline < period, CBS enables a task to run for more than the
> - * runtime / period. In a very loaded system, this can cause a domino
> - * effect, making other tasks miss their deadlines.
> - *
> - * To avoid this problem, in the activation of a constrained deadline
> - * task after the deadline but before the next period, throttle the
> - * task and set the replenishing timer to the begin of the next period,
> - * unless it is boosted.
> + * XXX: Daniel will document it better in a clean patch, this is
> + * a proof of concept.
>   */
>  static inline void dl_check_constrained_dl(struct sched_dl_entity *dl_se)
>  {
>  	struct task_struct *p = dl_task_of(dl_se);
>  	struct rq *rq = rq_of_dl_rq(dl_rq_of_se(dl_se));
> +	u64 clock = rq_clock(rq);
>  
> -	if (dl_time_before(dl_se->deadline, rq_clock(rq)) &&
> -	    dl_time_before(rq_clock(rq), dl_next_period(dl_se))) {
> -		if (unlikely(dl_se->dl_boosted || !start_dl_timer(p)))
> -			return;
> -		dl_se->dl_throttled = 1;
> +	/* we are in a new period */
> +	if (dl_time_before(dl_next_period(dl_se), rq_clock(rq)))
> +		return;
> +
> +	/* are we ok with the deadline and density? */
> +	if (dl_time_before(rq_clock(rq), dl_se->deadline) &&
> +	    !dl_entity_overflow(dl_se, dl_se, rq_clock(rq)))
> +		return;
> +
> +	/* is the density the problem? */
> +	if (dl_entity_overflow(dl_se, dl_se, clock)) {
> +		/* reduces the runtime so it fits in the density */
> +		dl_se->runtime =
> +		  (div64_u64(dl_se->dl_runtime << 20, dl_se->dl_deadline) *
> +		  (dl_se->deadline - clock)) >> 20;
> +		WARN_ON(dl_se->runtime < 0);
> +		return;
>  	}
> +
> +	if (!start_dl_timer(p))
> +		return;
> +
> +	dl_se->dl_throttled = 1;
>  }
>  
>  static
> @@ -996,8 +1000,11 @@ static void enqueue_task_dl(struct rq *rq, struct task_struct *p, int flags)
>  	 * If that is the case, the task will be throttled and
>  	 * the replenishment timer will be set to the next period.
>  	 */
> -	if (!dl_se->dl_throttled && dl_is_constrained(dl_se))
> -		dl_check_constrained_dl(dl_se);
> +	if (!p->dl.dl_boosted &&
> +	    !p->dl.dl_throttled &&
> +	    dl_is_constrained(&p->dl))
> +		dl_check_constrained_dl(&p->dl);
> +
>  
>  	/*
>  	 * If p is throttled, we do nothing. In fact, if it exhausted