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Date: Fri, 24 Jul 2009 21:04:46 +0800
Message-ID: <454c71700907240604h4673f117j8ed58b9f2ee54798@mail.gmail.com>
Subject: Re: report a bug about sched_rt
From: sen wang <wangsen.linux@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mingo@elte.hu, akpm@linux-foundation.org, kernel@kolivas.org,
       npiggin@suse.de, arjan@infradead.org,
       linux-arm-kernel@lists.arm.linux.org.uk, linux-kernel@vger.kernel.org
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Linux is used in many fieldes. SCHED_OTHER tasks is important to
embedded system.

if there is a running state task（a realtime task）, how can we shcedule
the idle task up?
It is ridiculous！

since the throttle has a bug, why not fix it?

we just modify the codes of  checking conditions of picking rt taskes!

static struct task_struct *pick_next_task_rt(struct rq *rq)
{
 ...

       if (rt_rq_throttled(rt_rq)&& rq->cfs.nr_running)
               return NULL;

 ...
}

2009/7/24 Peter Zijlstra <peterz@infradead.org>:
> On Fri, 2009-07-24 at 18:57 +0800, sen wang wrote:
>> I find something is wrong about sched_rt.
>>
>>  when I am debugging my system with rt_bandwidth_enabled, there is a
>> running realtime FIFO task in the sched_rt running queue and
>>  the fair running queue is empty.  I found the idle task will be
>> scheduled up when the running task still lie in the  sched_rt running
>> queue!
>>
>> this will happen when rt runqueue passed it's rt_bandwidth_enabled
>> runtime,then the scheduler choose the idle task instead of realtime
>> FIFO task.
>>
>> the reason lie in: when scheduler try to pick up a realtime FIFO task,
>> it will check if rt_throttled is enabled,
>> if so, it'll return and try fair queue but it is empty, then it come
>> to the sched_idle class.
>>
>> I don't think it reasonable, we should give the realtime FIFO task the
>> chance, even when rt runqueue passed it's runtime.
>> because it is cpu's free time.
>>
>> To fix it ,and keep rt_bandwidth works as before, I think
>> pick_next_task_rt() is the best space,
>
> RT is about determinism, sometimes having some extra time dependent on
> the runnability of SCHED_OTHER tasks is utterly useless.
>
> If you don't like the throttle, disable it.
>
>
>
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