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    SubjectRe: [PATCH] sched: remove starvation in check_preempt_equal_prio()
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    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;
    >>  }
    >
    >
    >
    --
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