Re: [sched-devel, patch-rfc] rework #2 of "prioritize non-migratable tasks over migratable ones"

["Gregory Haskins" <ghaskins@novell.com>]

>>> On Mon, Jul 14, 2008 at 10:50 AM, in message
<b647ffbd0807140750r54e5139aj7b25a28ef7ca247e@mail.gmail.com>, "Dmitry
Adamushko" <dmitry.adamushko@gmail.com> wrote: 
> 2008/7/1 Dmitry Adamushko <dmitry.adamushko@gmail.com>:
>>
>> Finally, this new version compiles and boots (applied a minor compilation 
> fix).
>> Functionality is not yet fully tested though.
>>
>> It's on top of today's tip tree.
>>
>> Any objections to this approach?
> 
> [ ping ]
> 
> Gregory or anyone else, any feedback on this one?

Hi Dmitry,
  A brief inspection of the patch looks good.  I am still not convinced we need to worry
about this condition anymore, however.  The fact that we cannot guarantee that the
migratable task will start running seems that it violates *something*, even if it is not
technically called out in posix. :)

But if we are going to solve that problem, this seems like a decent way to do it.

Regards,
-Greg

> 
> TIA,
> 
> 
>>
>> ---
>> From: Dmitry Adamushko <dmitry.adamushko@gmail.com>
>> Subject: prioritize non-migratable tasks over migratable ones in a generic 
> way
>>
>> (1) handle in a generic way all cases when a newly woken-up task is
>> not migratable (not just a corner case when "rt_se->nr_cpus_allowed ==
>> 1")
>>
>> (2) if current is to be preempted, then make sure "p" will be picked
>> up by pick_next_task_rt().
>> i.e. move task's group at the head of its list as well.
>>
>> currently, it's not a case for the group-scheduling case as described
>> here: http://www.ussg.iu.edu/hypermail/linux/kernel/0807.0/0134.html
>>
>>
>> Signed-off-by: Dmitry Adamushko <dmitry.adamushko@gmail.com>
>>
>> ---
>> diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
>> index 7c96147..7bc73e8 100644
>> --- a/kernel/sched_rt.c
>> +++ b/kernel/sched_rt.c
>> @@ -599,11 +599,7 @@ static void __enqueue_rt_entity(struct sched_rt_entity 
> *rt_se)
>>        if (group_rq && (rt_rq_throttled(group_rq) || !group_rq->rt_nr_running))
>>                return;
>>
>> -       if (rt_se->nr_cpus_allowed == 1)
>> -               list_add(&rt_se->run_list, queue);
>> -       else
>> -               list_add_tail(&rt_se->run_list, queue);
>> -
>> +       list_add_tail(&rt_se->run_list, queue);
>>        __set_bit(rt_se_prio(rt_se), array->bitmap);
>>
>>        inc_rt_tasks(rt_se, rt_rq);
>> @@ -689,31 +685,33 @@ static void dequeue_task_rt(struct rq *rq, struct 
> task_struct *p, int sleep)
>>  * followed by enqueue.
>>  */
>>  static
>> -void requeue_rt_entity(struct rt_rq *rt_rq, struct sched_rt_entity *rt_se)
>> +void requeue_rt_entity(struct rt_rq *rt_rq, struct sched_rt_entity *rt_se, 
> int head)
>>  {
>> -       struct rt_prio_array *array = &rt_rq->active;
>> -
>>        if (on_rt_rq(rt_se)) {
>> -               list_del_init(&rt_se->run_list);
>> -               list_add_tail(&rt_se->run_list,
>> -                             array->queue + rt_se_prio(rt_se));
>> +               struct rt_prio_array *array = &rt_rq->active;
>> +               struct list_head *queue = array->queue + rt_se_prio(rt_se);
>> +
>> +               if (head)
>> +                       list_move(&rt_se->run_list, queue);
>> +               else
>> +                       list_move_tail(&rt_se->run_list, queue);
>>        }
>>  }
>>
>> -static void requeue_task_rt(struct rq *rq, struct task_struct *p)
>> +static void requeue_task_rt(struct rq *rq, struct task_struct *p, int head)
>>  {
>>        struct sched_rt_entity *rt_se = &p->rt;
>>        struct rt_rq *rt_rq;
>>
>>        for_each_sched_rt_entity(rt_se) {
>>                rt_rq = rt_rq_of_se(rt_se);
>> -               requeue_rt_entity(rt_rq, rt_se);
>> +               requeue_rt_entity(rt_rq, rt_se, head);
>>        }
>>  }
>>
>>  static void yield_task_rt(struct rq *rq)
>>  {
>> -       requeue_task_rt(rq, rq->curr);
>> +       requeue_task_rt(rq, rq->curr, 0);
>>  }
>>
>>  #ifdef CONFIG_SMP
>> @@ -753,6 +751,30 @@ static int select_task_rq_rt(struct task_struct *p, int 
> sync)
>>         */
>>        return task_cpu(p);
>>  }
>> +
>> +static void check_preempt_equal_prio(struct rq *rq, struct task_struct *p)
>> +{
>> +       cpumask_t mask;
>> +
>> +       if (rq->curr->rt.nr_cpus_allowed == 1)
>> +               return;
>> +
>> +       if (p->rt.nr_cpus_allowed != 1
>> +           && cpupri_find(&rq->rd->cpupri, p, &mask))
>> +               return;
>> +
>> +       if (!cpupri_find(&rq->rd->cpupri, rq->curr, &mask))
>> +               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);
>> +       resched_task(rq->curr);
>> +}
>> +
>>  #endif /* CONFIG_SMP */
>>
>>  /*
>> @@ -778,18 +800,8 @@ static void check_preempt_curr_rt(struct rq *rq, struct 
> task_struct *p)
>>         * to move current somewhere else, making room for our non-migratable
>>         * task.
>>         */
>> -       if((p->prio == rq->curr->prio)
>> -          && p->rt.nr_cpus_allowed == 1
>> -          && rq->curr->rt.nr_cpus_allowed != 1) {
>> -               cpumask_t mask;
>> -
>> -               if (cpupri_find(&rq->rd->cpupri, rq->curr, &mask))
>> -                       /*
>> -                        * There appears to be other cpus that can accept
>> -                        * current, so lets reschedule to try and push it 
> away
>> -                        */
>> -                       resched_task(rq->curr);
>> -       }
>> +       if (p->prio == rq->curr->prio && !need_resched())
>> +               check_preempt_equal_prio(rq, p);
>>  #endif
>>  }
>>
>> @@ -1415,7 +1427,7 @@ static void task_tick_rt(struct rq *rq, struct 
> task_struct *p, int queued)
>>         * on the queue:
>>         */
>>        if (p->rt.run_list.prev != p->rt.run_list.next) {
>> -               requeue_task_rt(rq, p);
>> +               requeue_task_rt(rq, p, 0);
>>                set_tsk_need_resched(p);
>>        }
>>  }
>>
>> ---
>>
>>
>> --Dmitry
>>
>>
>>