Re: [RFC] sched/deadline: Prevent rt_time growth to infinity

[Kirill Tkhai <tkhai@yandex.ru>]

On 21.02.2014 20:36, Juri Lelli wrote:
> On Fri, 21 Feb 2014 11:37:15 +0100
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
>> On Thu, Feb 20, 2014 at 02:16:00AM +0400, Kirill Tkhai wrote:
>>> Since deadline tasks share rt bandwidth, we must care about
>>> bandwidth timer set. Otherwise rt_time may grow up to infinity
>>> in update_curr_dl(), if there are no other available RT tasks
>>> on top level bandwidth.
>>>
>>> I'm going to decide the problem the way below. Almost untested
>>> because of I skipped almost all of recent patches which haveto be applied from lkml.
>>>
>>> Please say, if I skipped anything in idea. Maybe better put
>>> start_top_rt_bandwidth() into set_curr_task_dl()?
>>
>> How about we only increment rt_time when there's an RT bandwidth timer
>> active?
>>
>>
>> ---
>> --- a/kernel/sched/rt.c
>> +++ b/kernel/sched/rt.c
>> @@ -568,6 +568,12 @@ static inline struct rt_bandwidth *sched
>>  
>>  #endif /* CONFIG_RT_GROUP_SCHED */
>>  
>> +bool sched_rt_bandwidth_active(struct rt_rq *rt_rq)
>> +{
>> +	struct rt_bandwidth *rt_b = sched_rt_bandwidth(rt_rq);
>> +	return hrtimer_active(&rt_b->rt_period_timer);
>> +}
>> +
>>  #ifdef CONFIG_SMP
>>  /*
>>   * We ran out of runtime, see if we can borrow some from our neighbours.
>> --- a/kernel/sched/deadline.c
>> +++ b/kernel/sched/deadline.c
>> @@ -587,6 +587,8 @@ int dl_runtime_exceeded(struct rq *rq, s
>>  	return 1;
>>  }
>>  
>> +extern bool sched_rt_bandwidth_active(struct rt_rq *rt_rq);
>> +
>>  /*
>>   * Update the current task's runtime statistics (provided it is still
>>   * a -deadline task and has not been removed from the dl_rq).
>> @@ -650,11 +652,13 @@ static void update_curr_dl(struct rq *rq
>>  		struct rt_rq *rt_rq = &rq->rt;
>>  
>>  		raw_spin_lock(&rt_rq->rt_runtime_lock);
>> -		rt_rq->rt_time += delta_exec;
>>  		/*
>>  		 * We'll let actual RT tasks worry about the overflow here, we
>> -		 * have our own CBS to keep us inline -- see above.
>> +		 * have our own CBS to keep us inline; only account when RT
>> +		 * bandwidth is relevant.
>>  		 */
>> +		if (sched_rt_bandwidth_active(rt_rq))
>> +			rt_rq->rt_time += delta_exec;
>>  		raw_spin_unlock(&rt_rq->rt_runtime_lock);
>>  	}
>>  }
> 
> So, I ran some tests with the above and I'd like to share with you what
> I've found. You can find here a trace-cmd trace that should be feeded
> to kernelshark to be able to understand what follows (or feel free to
> reproduce same scenario :)):
> http://retis.sssup.it/~jlelli/traces/trace_rt_time.dat
> 
> Here you have a DL task (4/10) and a while(1) RT task, both running
> inside a rt_bw of 0.5. RT tasks is activated 500ms after DL. As I
> filtered in sched_rt_period_timer(), you can search for time instants
> when the rt_bw is replenished. It is evident that the first time after
> rt timer is activated back (search for start_bandwidth_timer), we can
> eat some bw to FAIR tasks (if any). This is due to the fact that we
> reset rt_bw budget at this time, start decrementing rt_time for both DL
> and RT tasks, throttle RT tasks when rt_time > runtime, but, since DL
> tasks acually executes inside their own server, they don't care about
> rt_bw. Good news is that steady state is ok: keeping track of overruns
> we are able to stop eating bw to other guys.
> 
> My thougths:
> 
>  - Peter's patch is an easy fix to Kirill's problem (RT tasks were
>    throttled too early);
>  - something to add to this solution could be to pre-calculate bw of
>    ready DL tasks and subtract it to rt_bw at replenishment time, but
>    it sounds quite awkward, pessimistic, and I'm not sure it is gonna
>    work;
>  - we are stealing bw to best-effort tasks, and just at the beginning
>    of the transistion, is it really a problem?
>  - I mean, if you want guarantees make your tasks DL! :);
>  - in the long run we are gonna have RT tasks scheduled inside CBS
>    servers, and all this will be properly fixed up.
> 
> Comments?
> 
> BTW, rt timer activation/deactivation should probably be fixed for
> !RT_GROUP_SCHED with something like this:
> 
> ---
>  kernel/sched/rt.c |   10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 6161de8..274f992 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -86,12 +86,12 @@ void init_rt_rq(struct rt_rq *rt_rq, struct rq *rq)
>  	raw_spin_lock_init(&rt_rq->rt_runtime_lock);
>  }
>  
> -#ifdef CONFIG_RT_GROUP_SCHED
>  static void destroy_rt_bandwidth(struct rt_bandwidth *rt_b)
>  {
>  	hrtimer_cancel(&rt_b->rt_period_timer);
>  }
>  
> +#ifdef CONFIG_RT_GROUP_SCHED
>  #define rt_entity_is_task(rt_se) (!(rt_se)->my_q)
>  
>  static inline struct task_struct *rt_task_of(struct sched_rt_entity *rt_se)
> @@ -1017,8 +1017,12 @@ inc_rt_group(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
>  	start_rt_bandwidth(&def_rt_bandwidth);
>  }
>  
> -static inline
> -void dec_rt_group(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq) {}
> +static void
> +dec_rt_group(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
> +{
> +	if (!rt_rq->rt_nr_running)
> +		destroy_rt_bandwidth(&def_rt_bandwidth);
> +}
>  
>  #endif /* CONFIG_RT_GROUP_SCHED */
>  

It looks with both patches applied, we may get into a situation,
when all CPU time is shared between RT and DL tasks:

rt_runtime = n
rt_period  = 2n

| RT working, DL sleeping  | DL working, RT sleeping      |
-----------------------------------------------------------
| (1)     duration = n     | (2)     duration = n         | (repeat)
|--------------------------|------------------------------|
| (rt_bw timer is running) | (rt_bw timer is not running) |

No time for fair tasks at all.

Kirill