Re: [PATCH RESEND v4 2/3] sched/rt: Fix wrong SMP scheduler behavior for equal prio cases

[Xunlei Pang]

On 27 March 2015 at 23:28, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> On Mon,  9 Mar 2015 15:32:27 +0800
> Xunlei Pang <xlpang@xxxxxxx> wrote:
>
>> From: Xunlei Pang <pang.xunlei@xxxxxxxxxx>
>>
>> Currently, SMP RT scheduler has some trouble in dealing with
>> equal prio cases.
>>
>> For example, in check_preempt_equal_prio():
>> When RT1(current task) gets preempted by RT2, if there is a
>> migratable RT3 with same prio, RT3 will be pushed away instead
>> of RT1 afterwards, because RT1 will be enqueued to the tail of
>> the pushable list when going through succeeding put_prev_task_rt()
>> triggered by resched. This broke FIFO.
>>
>> Furthermore, this is also problematic for normal preempted cases
>> if there're some rt tasks queued with the same prio as current.
>> Because current will be put behind these tasks in the pushable
>> queue.
>>
>> So, if a task is running and gets preempted by a higher priority
>> task (or even with same priority for migrating), this patch ensures
>> that it is put ahead of any existing task with the same priority in
>> the pushable queue.
>>
>> Signed-off-by: Xunlei Pang <pang.xunlei@xxxxxxxxxx>
>> ---
>>  kernel/sched/rt.c | 23 ++++++++++++++++-------
>>  1 file changed, 16 insertions(+), 7 deletions(-)
>>
>> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
>> index f4d4b07..86cd79f 100644
>> --- a/kernel/sched/rt.c
>> +++ b/kernel/sched/rt.c
>> @@ -347,11 +347,15 @@ static inline void set_post_schedule(struct rq *rq)
>>       rq->post_schedule = has_pushable_tasks(rq);
>>  }
>>
>> -static void enqueue_pushable_task(struct rq *rq, struct task_struct *p)
>> +static void enqueue_pushable_task(struct rq *rq,
>> +                             struct task_struct *p, bool head)
>
> Nit.
>
> static void
> enqueue_pushable_task(struct rq *rq, struct task_struct *p, bool head)
>
> Is a better breaking of the line.

I'll adjust it to be one line.

>
>>  {
>>       plist_del(&p->pushable_tasks, &rq->rt.pushable_tasks);
>>       plist_node_init(&p->pushable_tasks, p->prio);
>> -     plist_add(&p->pushable_tasks, &rq->rt.pushable_tasks);
>> +     if (head)
>> +             plist_add_head(&p->pushable_tasks, &rq->rt.pushable_tasks);
>> +     else
>> +             plist_add_tail(&p->pushable_tasks, &rq->rt.pushable_tasks);
>>
>>       /* Update the highest prio pushable task */
>>       if (p->prio < rq->rt.highest_prio.next)
>> @@ -373,7 +377,8 @@ static void dequeue_pushable_task(struct rq *rq, struct task_struct *p)
>>
>>  #else
>>
>> -static inline void enqueue_pushable_task(struct rq *rq, struct task_struct *p)
>> +static inline void enqueue_pushable_task(struct rq *rq,
>> +                                     struct task_struct *p, bool head)
>
> Same here.
>
>>  {
>>  }
>>
>> @@ -1248,7 +1253,7 @@ enqueue_task_rt(struct rq *rq, struct task_struct *p, int flags)
>>       enqueue_rt_entity(rt_se, flags & ENQUEUE_HEAD);
>>
>>       if (!task_current(rq, p) && p->nr_cpus_allowed > 1)
>> -             enqueue_pushable_task(rq, p);
>> +             enqueue_pushable_task(rq, p, false);
>>  }
>>
>>  static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int flags)
>> @@ -1494,8 +1499,12 @@ static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
>>        * The previous task needs to be made eligible for pushing
>>        * if it is still active
>>        */
>> -     if (on_rt_rq(&p->rt) && p->nr_cpus_allowed > 1)
>> -             enqueue_pushable_task(rq, p);
>> +     if (on_rt_rq(&p->rt) && p->nr_cpus_allowed > 1) {
>> +             if (task_running(rq, p) && (preempt_count() & PREEMPT_ACTIVE))
>
> put_prev_task_rt() is called by put_prev_task() which is called by
> several functions: rt_mutex_setprio(), __sched_setscheduler(),
> sched_setnuma(), migrate_tasks(), and sched_move_task(). It's not part
> of being preempted.
>
> Now it is also called by pick_next_task_rt() which I'm assuming is what
> you want it to affect.
>
> The above definitely needs a comment about what it is doing. Also, I'm
> not so sure we care about testing task_running(). I'm thinking the
> check for PREEMPT_ACTIVE is good enough, as that would only be set from
> being called within preempt_schedule().

Indeed.

>
> Also, we could get rid of the if statement and do:
>
>         enqueue_pushable_task(rq, p, !!(preempt_count() & PREEMPT_ACTIVE));

Agree, will do. Thanks.

>
>
> -- Steve
>
>> +                     enqueue_pushable_task(rq, p, true);
>> +             else
>> +                     enqueue_pushable_task(rq, p, false);
>> +     }
>>  }
>>
>>  #ifdef CONFIG_SMP
>> @@ -1914,7 +1923,7 @@ static void set_cpus_allowed_rt(struct task_struct *p,
>>               rq->rt.rt_nr_migratory--;
>>       } else {
>>               if (!task_current(rq, p))
>> -                     enqueue_pushable_task(rq, p);
>> +                     enqueue_pushable_task(rq, p, false);
>>               rq->rt.rt_nr_migratory++;
>>       }
>>
>
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