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 11:28:04 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Xunlei Pang <xlpang@....com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...il.com>,
	Xunlei Pang <pang.xunlei@...aro.org>
Subject: Re: [PATCH RESEND v4 2/3] sched/rt: Fix wrong SMP scheduler
 behavior for equal prio cases

On Mon,  9 Mar 2015 15:32:27 +0800
Xunlei Pang <xlpang@....com> wrote:

> 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)

Nit.

static void
enqueue_pushable_task(struct rq *rq, struct task_struct *p, bool head)

Is a better breaking of the 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().

Also, we could get rid of the if statement and do:

	enqueue_pushable_task(rq, p, !!(preempt_count() & PREEMPT_ACTIVE));


-- 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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