Re: [PATCH] sched: remove starvation in check_preempt_equal_prio()

[Hillf Danton <dhillf@gmail.com>]

On Tue, May 24, 2011 at 9:45 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
> On Tue, 2011-05-24 at 21:34 +0800, Hillf Danton wrote:
>> If there are pushable tasks and they are high enough in priority, in which
>> case task p is covered, the current could keep holding its CPU.
>>
>> Even if current task has to release its CPU, requeuing task p could result in
>> starvation of tasks that are of same priority and have been waiting on RQ for
>> a couple of hours:/
>
> Can you explain this better? Sounds like you are describing the
> definition of FIFO. You are *not* suppose to preempt a FIFO task just

I dont want to redefine FIFO, but starvation needs attention, since
the woken task is already off RQ, and its position on RQ is reshuffled.

thanks
          Hillf
> because another task of equal priority woke up on its run queue.
>
> Yes, if you queue two FIFO tasks of the same priority on the same run
> queue, and one runs for hours without calling schedule. The other one
> will have to wait.
>
> -- Steve
>
>
>>
>> Signed-off-by: Hillf Danton <dhillf@gmail.com>
>> ---
>>
>> --- tip-git/kernel/sched_rt.c Sun May 22 20:12:01 2011
>> +++ sched_rt.c        Tue May 24 21:01:51 2011
>> @@ -1028,24 +1028,23 @@ select_task_rq_rt(struct task_struct *p,
>>       return cpu;
>>  }
>>
>> +static struct task_struct *pick_next_pushable_task(struct rq *);
>> +
>>  static void check_preempt_equal_prio(struct rq *rq, struct task_struct *p)
>>  {
>> -     if (rq->curr->rt.nr_cpus_allowed == 1)
>> -             return;
>> -
>> -     if (p->rt.nr_cpus_allowed != 1
>> -         && cpupri_find(&rq->rd->cpupri, p, NULL))
>> +     if (rq->curr->rt.nr_cpus_allowed > 1) {
>> +             struct task_struct *push = pick_next_pushable_task(rq);
>> +             /*
>> +              * Though curr is pushable, if there are other pushable tasks,
>> +              * we keep curr busy.
>> +              */
>> +             if (push && !(push->prio > p->prio))
>> +                     return;
>> +     } else
>>               return;
>>
>> -     if (!cpupri_find(&rq->rd->cpupri, rq->curr, NULL))
>> -             return;
>> -
>> -     /*
>> -      * There appears to be other cpus that can accept
>> -      * current and none to run 'p', so lets reschedule
>> -      * to try and push current away:
>> -      */
>> -     requeue_task_rt(rq, p, 1);
>> +     /* yield curr */
>> +     requeue_task_rt(rq, rq->curr, 0);
>>       resched_task(rq->curr);
>>  }
>>
>> @@ -1091,7 +1090,7 @@ static struct sched_rt_entity *pick_next
>>       BUG_ON(idx >= MAX_RT_PRIO);
>>
>>       queue = array->queue + idx;
>> -     next = list_entry(queue->next, struct sched_rt_entity, run_list);
>> +     next = list_first_entry(queue, struct sched_rt_entity, run_list);
>>
>>       return next;
>>  }
>
>
>