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SubjectRe: [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@126.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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