linux-kernel - Re: [PATCH v6 2/3] sched/rt: Fix wrong SMP scheduler behavior for equal prio cases

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Message-ID: <20150420105228.461e103c@gandalf.local.home>
Date:	Mon, 20 Apr 2015 10:52:28 -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 v6 2/3] sched/rt: Fix wrong SMP scheduler behavior for
 equal prio cases

On Mon, 20 Apr 2015 16:22:47 +0800
Xunlei Pang <xlpang@....com> wrote:

>  static inline void enqueue_pushable_task(struct rq *rq, struct task_struct *p)
>  {
>  }
> @@ -1506,8 +1526,21 @@ 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) {
> +		/*
> +		 * put_prev_task_rt() is called by many functions,
> +		 * pick_next_task_rt() is the only one may have
> +		 * PREEMPT_ACTIVE set. So if detecting p(current
> +		 * task) is preempted in such case, we should
> +		 * enqueue it to the front of the pushable plist,
> +		 * as there may be multiple tasks with the same
> +		 * priority as p.

The above comment is very difficult to understand. Maybe something like:

		/*
		 * When put_prev_task_rt() is called by
		 * pick_next_task_rt(), if PREEMPT_ACTIVE is set, it
		 * means that the current rt task is being preempted by
		 * a higher priority task. To maintain FIFO, it must
		 * stay ahead of any other task that is queued at the
		 * same priority.
		 */

-- Steve

> +		 */
> +		if (preempt_count() & PREEMPT_ACTIVE)
> +			enqueue_pushable_task_preempted(rq, p);
> +		else
> +			enqueue_pushable_task(rq, p);
> +	}
>  }
>  
>  #ifdef CONFIG_SMP

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