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Date: Sat, 22 Feb 2014 04:56:59 +0400
From: Kirill Tkhai <tkhai@...dex.ru>
To: Juri Lelli <juri.lelli@...il.com>,
Peter Zijlstra <peterz@...radead.org>
CC: "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
On 21.02.2014 20:36, Juri Lelli 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
> 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
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