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Reply-To: xlpang@redhat.com
Subject: Re: [PATCH] sched/deadline: Throttle a constrained task activated if
 overflow
References: <1491816131-20268-1-git-send-email-xlpang@redhat.com>
 <4d24b94c-1e57-b82c-ff0f-099157ba5526@redhat.com>
To: Daniel Bristot de Oliveira <bristot@redhat.com>,
        Xunlei Pang <xlpang@redhat.com>, linux-kernel@vger.kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>, Juri Lelli <juri.lelli@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Luca Abeni <luca.abeni@santannapisa.it>,
        Steven Rostedt <rostedt@goodmis.org>,
        Tommaso Cucinotta <tommaso.cucinotta@sssup.it>,
        =?UTF-8?Q?R=c3=b4mulo_Silva_de_Oliveira?= 
        <romulo.deoliveira@ufsc.br>,
        Mathieu Poirier <mathieu.poirier@linaro.org>
From: Xunlei Pang <xpang@redhat.com>
Message-ID: <58EDBAC4.7020902@redhat.com>
Date: Wed, 12 Apr 2017 13:27:32 +0800
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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)
>
> 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
>
> Voilà. 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;

The more I read the code, the more I am confused why dl_entity_overflow() is needed, if the
task is before its deadline, just let it run. E.g. For (runtime 2ms, deadline 4ms, period 8ms),
for some reason was preempted after running a very short time 0.1ms, after 0.9ms it was
scheduled back and immediately sleep 1ms, when it is awakened, remaining runtime is 1.9ms,
remaining deadline(deadline-now) within this period is 2ms, but dl_entity_overflow() is true.
However, clearly it can be correctly run 1.9ms before deadline comes wthin this period.

We can add a condition in dl_runtime_exceeded(), if its deadline is passed, then zero
its runtime if positive, and a new period begins.

I did some tests with the following patch, seems it works well, please correct me if I am wrong.
---
 kernel/sched/deadline.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index a2ce590..600c530 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -498,8 +498,7 @@ static void update_dl_entity(struct sched_dl_entity *dl_se,
     struct dl_rq *dl_rq = dl_rq_of_se(dl_se);
     struct rq *rq = rq_of_dl_rq(dl_rq);
 
-    if (dl_time_before(dl_se->deadline, rq_clock(rq)) ||
-        dl_entity_overflow(dl_se, pi_se, rq_clock(rq))) {
+    if (!dl_time_before(rq_clock(rq), dl_se->deadline)) {
         dl_se->deadline = rq_clock(rq) + pi_se->dl_deadline;
         dl_se->runtime = pi_se->dl_runtime;
     }
@@ -722,13 +721,22 @@ static inline void dl_check_constrained_dl(struct sched_dl_entity *dl_se)
         dl_time_before(rq_clock(rq), dl_next_period(dl_se))) {
         if (unlikely(dl_se->dl_boosted || !start_dl_timer(p)))
             return;
+
+        if (dl_se->runtime > 0)
+            dl_se->runtime = 0;
+
         dl_se->dl_throttled = 1;
     }
 }
 
 static
-int dl_runtime_exceeded(struct sched_dl_entity *dl_se)
+int dl_runtime_exceeded(struct rq *rq, struct sched_dl_entity *dl_se)
 {
+    if (!dl_time_before(rq_clock(rq), dl_se->deadline)) {
+        if (dl_se->runtime > 0)
+            dl_se->runtime = 0;
+    }
+
     return (dl_se->runtime <= 0);
 }
 
@@ -779,7 +787,7 @@ static void update_curr_dl(struct rq *rq)
     dl_se->runtime -= delta_exec;
 
 throttle:
-    if (dl_runtime_exceeded(dl_se) || dl_se->dl_yielded) {
+    if (dl_runtime_exceeded(rq, dl_se) || dl_se->dl_yielded) {
         dl_se->dl_throttled = 1;
         __dequeue_task_dl(rq, curr, 0);
         if (unlikely(dl_se->dl_boosted || !start_dl_timer(curr)))
-- 
1.8.3.1