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SubjectRe: [patch 2/8] track highest prio queued on runqueue
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On Fri, 2007-10-19 at 14:42 -0400, Steven Rostedt wrote:
> plain text document attachment (add-rq-highest-prio.patch)
> This patch adds accounting to each runqueue to keep track of the
> highest prio task queued on the run queue. We only care about
> RT tasks, so if the run queue does not contain any active RT tasks
> its priority will be considered MAX_RT_PRIO.
>
> This information will be used for later patches.
>
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
>
> ---
> kernel/sched.c | 7 +++++++
> kernel/sched_rt.c | 25 +++++++++++++++++++++++++
> 2 files changed, 32 insertions(+)
>
> Index: linux-test.git/kernel/sched.c
> ===================================================================
> --- linux-test.git.orig/kernel/sched.c 2007-10-19 12:33:09.000000000 -0400
> +++ linux-test.git/kernel/sched.c 2007-10-19 12:34:32.000000000 -0400
> @@ -324,6 +324,8 @@ struct rq {
> int push_cpu;
> /* cpu of this runqueue: */
> int cpu;
> + /* highest queued rt task prio */
> + int highest_prio;
>
> struct task_struct *migration_thread;
> struct list_head migration_queue;
> @@ -972,6 +974,8 @@ static void activate_task(struct rq *rq,
>
> enqueue_task(rq, p, wakeup);
> inc_nr_running(p, rq);
> +
> + rq_prio_add_task(rq, p);
> }

We are better off doing this in enqueue_task() or PI boosting will fail
to alter the prio of the RQ.

>
> /*
> @@ -984,6 +988,8 @@ static void deactivate_task(struct rq *r
>
> dequeue_task(rq, p, sleep);
> dec_nr_running(p, rq);
> +
> + rq_prio_remove_task(rq, p);

Ditto for dequeue_task()

> }
>
> /**
> @@ -6619,6 +6625,7 @@ void __init sched_init(void)
> rq->cpu = i;
> rq->migration_thread = NULL;
> INIT_LIST_HEAD(&rq->migration_queue);
> + rq->highest_prio = MAX_RT_PRIO;
> #endif
> atomic_set(&rq->nr_iowait, 0);
>
> Index: linux-test.git/kernel/sched_rt.c
> ===================================================================
> --- linux-test.git.orig/kernel/sched_rt.c 2007-10-19 12:33:09.000000000 -0400
> +++ linux-test.git/kernel/sched_rt.c 2007-10-19 12:33:23.000000000 -0400
> @@ -110,6 +110,31 @@ static struct task_struct *pick_next_tas
> return next;
> }
>
> +#ifdef CONFIG_SMP
> +static inline void rq_prio_add_task(struct rq *rq, struct task_struct *p)
> +{
> + if (unlikely(rt_task(p)) && p->prio < rq->highest_prio)
> + rq->highest_prio = p->prio;
> +}
> +
> +static inline void rq_prio_remove_task(struct rq *rq, struct task_struct *p)
> +{
> + struct rt_prio_array *array;
> +
> + if (unlikely(rt_task(p))) {
> + if (rq->rt_nr_running) {
> + if (p->prio >= rq->highest_prio) {

<= ?

We only need to recalc if the task being removed was the highest prio
(or higher, if that were possible but it shouldnt be).

I think this logic will technically work as-is because every task is
always >= highest in a properly accounted system, so you will just
simply recalc for every remove. But I do like the optimization that you
were trying to add.

So for the paranoid:

BUG_ON(p->prio < rq->highest_prio);
if (p->prio == rq->highest_prio) {
/* recalc */
}

> + /* recalculate */
> + array = &rq->rt.active;
> + rq->highest_prio =
> + sched_find_first_bit(array->bitmap);
> + } /* otherwise leave rq->highest prio alone */
> + } else
> + rq->highest_prio = MAX_RT_PRIO;
> + }
> +}
> +#endif /* CONFIG_SMP */
> +
> static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
> {
> update_curr_rt(rq);
>
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