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SubjectRe: [PATCH] sched: check and preempt the current task at right place
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On Tue, 2011-05-31 at 21:04 +0800, Hillf Danton wrote:
> When switching a task to RT, it is the right place to preempt the current
> running task if the task is not running. Same scenario also rises when the
> priority of task has changed.
>
> Signed-off-by: Hillf Danton <dhillf@gmail.com>
> ---
> kernel/sched_rt.c | 19 +++----------------
> 1 files changed, 3 insertions(+), 16 deletions(-)
>
> diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
> index 88725c9..4a9553e 100644
> --- a/kernel/sched_rt.c
> +++ b/kernel/sched_rt.c
> @@ -1671,25 +1671,13 @@ static inline void init_sched_rt_class(void)
> */
> static void switched_to_rt(struct rq *rq, struct task_struct *p)
> {
> - int check_resched = 1;
> -
> /*
> * If we are already running, then there's nothing
> * that needs to be done. But if we are not running
> * we may need to preempt the current running task.
> - * If that current running task is also an RT task
> - * then see if we can move to another run queue.
> */
> - if (p->on_rq && rq->curr != p) {
> -#ifdef CONFIG_SMP
> - if (rq->rt.overloaded && push_rt_task(rq) &&
> - /* Don't resched if we changed runqueues */
> - rq != task_rq(p))
> - check_resched = 0;
> -#endif /* CONFIG_SMP */
> - if (check_resched && p->prio < rq->curr->prio)
> - resched_task(rq->curr);
> - }
> + if (p->on_rq && rq->curr != p)
> + check_preempt_curr_rt(rq, p, 0);


Note, although it works now, there's no guarantee that
check_preempt_curr_rt() will work with something other than an rt task
on the runqueue.

Also, this code removes the push_rt_task(rq) entirely, which means that
if we don't preempt current, and p is an RT task (as it is), we wont
push it off to another rq even if one is available for it to run on.

-- Steve


> }
>
> /*
> @@ -1729,8 +1717,7 @@ prio_changed_rt(struct rq *rq, struct
> task_struct *p, int oldprio)
> * greater than the current running task
> * then reschedule.
> */
> - if (p->prio < rq->curr->prio)
> - resched_task(rq->curr);
> + check_preempt_curr_rt(rq, p, 0);
> }
> }




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