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SubjectRe: [PATCH v5 2/3] sched/rt: Fix wrong SMP scheduler behavior for equal prio cases
On Thu,  9 Apr 2015 11:27:17 +0800
Xunlei Pang <xlpang@126.com> wrote:


> Suggested-by: Steven Rostedt <rostedt@goodmis.org>
> Signed-off-by: Xunlei Pang <pang.xunlei@linaro.org>
> ---
> kernel/sched/rt.c | 26 ++++++++++++++++++++------
> 1 file changed, 20 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 575da76..402162a 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -359,11 +359,15 @@ 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)
> @@ -385,7 +389,8 @@ 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)
> {
> }
>
> @@ -1260,7 +1265,7 @@ 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);

Hmm, I really don't like the "false" parameter all over the place, since
it's only needed in one place. Thinking about this more, what about
keeping enqueue_pushable_task() as is, and adding an
enqueue_pushable_task_preempted(). Having something like this:

static inline void
enqueue_pushable_task(struct rq *rq, struct task_struct *p)
{
enqueue_pushable_task_preempted(rq, p, false);
}


> }
>
> static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int flags)
> @@ -1507,7 +1512,16 @@ static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
> * if it is still active
> */
> if (on_rt_rq(&p->rt) && p->nr_cpus_allowed > 1)
> - enqueue_pushable_task(rq, p);
> + /*
> + * 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.
> + */
> + enqueue_pushable_task(rq, p, !!(preempt_count() & PREEMPT_ACTIVE));

Then we don't need to touch any of the code but this place, and this
would be:

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

I'm thinking this would be much more descriptive.

What do you think?

-- Steve

> }
> #ifdef CONFIG_SMP
> @@ -2091,7 +2105,7 @@ 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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