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Date: Sat, 22 Feb 2014 03:50:02 +0400
From: Kirill Tkhai <tkhai@...dex.ru>
To: Juri Lelli <juri.lelli@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Steven Rostedt <rostedt@...dmis.org>,
Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC] sched/deadline: Prevent rt_time growth to infinity
21.02.2014, 20:52, "Juri Lelli" <juri.lelli@...il.com>:
> On Fri, 21 Feb 2014 17:36:41 +0100
> Juri Lelli <juri.lelli@...il.com> wrote:
>
>> On Fri, 21 Feb 2014 11:37:15 +0100
>> Peter Zijlstra <peterz@...radead.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
>
> The reset happens when rt_bw replenishment timer fires, after a bit:
>
> sched_rt_period_timer <-- __run_hrtimer
Juri, sorry, I forgot to wrote I mean the situation when only one task is on_rq
at every moment.
DL, RT, DL, RT, ...
rt_runtime = n;
rt_period = 2n;
| DL's working, RT's sleeping | RT's working, DL's sleeping | all sleep |
------------------------------------------------------------------------------------------|
| (1) duration = n | (2) duration = n | (3) duration = n | (repeat)
|------------------------------|------------------------------|---------------------------|
| (rt_bw timer is not running) | (rt_bw timer is running) |
According to the patch, rt_bw timer is working only if we have queued RT task.
In the case above part (1) has no queued RT tasks, so timer is not working.
rt_time is not being increased too.
We have ratio 2/3.
Thanks,
Kirill
>
> Apologies,
>
> - Juri
>
>> 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 */
--
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