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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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