linux-kernel - Re: [PATCH] sched: check and preempt the current task at right place

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Date:	Sun, 5 Jun 2011 18:45:36 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>,
	Yong Zhang <yong.zhang0@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] sched: check and preempt the current task at right place

On Wed, Jun 1, 2011 at 4:31 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Tue, 2011-05-31 at 21:04 +0800, Hillf Danton wrote:
>> When switching a task to RT, it is the right place to preempt the current
>> running task if the task is not running. Same scenario also rises when the
>> priority of task has changed.
>>
>> Signed-off-by: Hillf Danton <dhillf@...il.com>
>> ---
>>  kernel/sched_rt.c |   19 +++----------------
>>  1 files changed, 3 insertions(+), 16 deletions(-)
>>
>> diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
>> index 88725c9..4a9553e 100644
>> --- a/kernel/sched_rt.c
>> +++ b/kernel/sched_rt.c
>> @@ -1671,25 +1671,13 @@ static inline void init_sched_rt_class(void)
>>   */
>>  static void switched_to_rt(struct rq *rq, struct task_struct *p)
>>  {
>> -     int check_resched = 1;
>> -
>>       /*
>>        * If we are already running, then there's nothing
>>        * that needs to be done. But if we are not running
>>        * we may need to preempt the current running task.
>> -      * If that current running task is also an RT task
>> -      * then see if we can move to another run queue.
>>        */
>> -     if (p->on_rq && rq->curr != p) {
>> -#ifdef CONFIG_SMP
>> -             if (rq->rt.overloaded && push_rt_task(rq) &&
>> -                 /* Don't resched if we changed runqueues */
>> -                 rq != task_rq(p))
>> -                     check_resched = 0;
>> -#endif /* CONFIG_SMP */
>> -             if (check_resched && p->prio < rq->curr->prio)
>> -                     resched_task(rq->curr);
>> -     }
>> +     if (p->on_rq && rq->curr != p)
>> +             check_preempt_curr_rt(rq, p, 0);
>
>
> Note, although it works now, there's no guarantee that
> check_preempt_curr_rt() will work with something other than an rt task
> on the runqueue.
>

If check_preempt_curr_rt has solid shortage to do its work, we could do more
for it later.

> Also, this code removes the push_rt_task(rq) entirely, which means that
> if we don't preempt current, and p is an RT task (as it is), we wont
> push it off to another rq even if one is available for it to run on.
>

If it is concerned that we fail to preempt the current task with a RT task, why
is preempt checked after pushing?

If we really fail to preempt, it is not too late to push the starved
tasks, though
starvation is eligible as you taught a while ago.

thanks
          Hillf

>>  }
>>
>>  /*
>> @@ -1729,8 +1717,7 @@ prio_changed_rt(struct rq *rq, struct
>> task_struct *p, int oldprio)
>>                * greater than the current running task
>>                * then reschedule.
>>                */
>> -             if (p->prio < rq->curr->prio)
>> -                     resched_task(rq->curr);
>> +             check_preempt_curr_rt(rq, p, 0);
>>       }
>>  }
>
>
>
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