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

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Date:	Fri, 27 Mar 2015 22:39:15 +0800
From:	Xunlei Pang <pang.xunlei@...aro.org>
To:	Xunlei Pang <xlpang@....com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Juri Lelli <juri.lelli@...il.com>
Subject: Re: [PATCH RESEND v4 2/3] sched/rt: Fix wrong SMP scheduler behavior
 for equal prio cases

Ping Steve

> From: Xunlei Pang <pang.xunlei@...aro.org>
>
> 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@...aro.org>
> ---
>  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)
>  {
>         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)
>  {
>  }
>
> @@ -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))
> +                       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++;
>         }
>
> --
> 1.9.1
>
>
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