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SubjectRe: [patch 2/8] track highest prio queued on runqueue
On 19/10/2007, Steven Rostedt <rostedt@goodmis.org> wrote:
> [ ... ]
>
===================================================================
> --- 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;

again, could it be moved to 'struct rt_rq' ?
(so we want to cache it as we don't want to trash more per-cpu bytes
calling smth like
if (!rt_nr_running) sched_find_first_bit() from other CPUs)


> @@ -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);
> }
>
> /*
> @@ -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);
> }

{enqueue,dequeue}_task_rt() would be more appropriate places.


> +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) {
> + /* 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 */
> +

(just a few thoughts)

we call sched_find_first_bit() in pick_next_task_rt() in the case when
rt_nr_running != 0.

So if we can tolerate the 'latency' of updating the 'highest_prio' ==
the interval of time between deactivate_task() and pick_next_task() in
schedule() then rq_prio_remove_task() would just need to do a single
thing:

/* No more RT tasks: */
if (!rt_nr_running)
highest_prio = MAX_RT_PRIO;

and then,

static struct task_struct *pick_next_task_rt(struct rq *rq)
{
struct rt_prio_array *array = &rq->rt.active;
struct task_struct *next;
struct list_head *queue;
int idx;
- idx = sched_find_first_bit(array->bitmap);
+ rq->highest_prio = idx = sched_find_first_bit(array->bitmap);

[ ... ]

additionally, if we can tolerate the 'latency' (of updating
highest_prio) == the worst case scheduling latency, then
rq_prio_add_task() is not necessary at all.


--
Best regards,
Dmitry Adamushko
-
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