On Mon, 2007-10-15 at 14:05 -0400, Steven Rostedt wrote:
> --
> On Fri, 12 Oct 2007, Gregory Haskins wrote:
>
> > There are three events that require consideration for redistributing RT
> > tasks:
> >
> > 1) When one or more higher-priority tasks preempts a lower-one from a
> > RQ
> > 2) When a lower-priority task is woken up on a RQ
> > 3) When a RQ downgrades its current priority
> >
> > Steve Rostedt's push_rt patch addresses (1). It hooks in right after
> > a new task has been switched-in. If this was the result of an RT
> > preemption, or if more than one task was awoken at the same time, we
> > can try to push some of those other tasks away.
> >
> > This patch addresses (2). When we wake up a task, we check to see
> > if it would preempt the current task on the queue. If it will not, we
> > attempt to find a better suited CPU (e.g. one running something lower
> > priority than the task being woken) and try to activate the task there.
> >
> > Finally, we have (3). In theory, we only need to balance_rt_tasks() if
> > the following conditions are met:
> > 1) One or more CPUs are in overload, AND
> > 2) We are about to switch to a task that lowers our priority.
> >
> > (3) will be addressed in a later patch.
> >
> > Signed-off-by: Gregory Haskins <[email protected]>
> > ---
> >
> > kernel/sched.c | 68 ++++++++++++++++++++++++++++++++++----------------------
> > 1 files changed, 41 insertions(+), 27 deletions(-)
> >
> > diff --git a/kernel/sched.c b/kernel/sched.c
> > index 62f9f0b..3c71156 100644
> > --- a/kernel/sched.c
> > +++ b/kernel/sched.c
> > @@ -1628,6 +1628,12 @@ out:
> > return ret;
> > }
> >
> > +/* Push all tasks that we can to other CPUs */
> > +static void push_rt_tasks(struct rq *this_rq)
> > +{
> > + while (push_rt_task(this_rq));
>
> Loop conditions like this must be written as:
>
> while (push_rt_task(this_rq))
> ;
>
> So we don't accidently put something inside the loop if we forget to add
> the semicolon, like:
>
> while (push_rt_task(this_rq)
>
> do_something_not_expected_to_loop();
>
> Of course you end your function after that and thus we would get an
> compile error if the semicolon were to be missing. But we might add
> code afterwards.
I'm not sure I really get what the difference is, but I don't feel
strongly one way or the other. I can make that change.
>
> > +}
> > +
> > /*
> > * Pull RT tasks from other CPUs in the RT-overload
> > * case. Interrupts are disabled, local rq is locked.
> > @@ -1988,7 +1994,33 @@ out_set_cpu:
> > this_cpu = smp_processor_id();
> > cpu = task_cpu(p);
> > }
> > -
> > +
> > + /*
> > + * If a newly woken up RT task cannot preempt the
> > + * current (RT) task (on a target runqueue) then try
> > + * to find another CPU it can preempt:
> > + */
> > + if (rt_task(p) && !TASK_PREEMPTS_CURR(p, rq)) {
> > + cpumask_t cpu_mask;
> > + cpus_and(cpu_mask, cpu_online_map, p->cpus_allowed);
>
> Hmm, maybe I should put that mask into the find_lowest_cpu function.
> Of course I changed this a little in my last patch.
If you have an update, send it along and I will rebase.
>
> > +
> > + new_cpu = find_lowest_cpu(&cpu_mask, p, rq);
> > + if ((new_cpu != -1) && (new_cpu != cpu)) {
> > + set_task_cpu(p, new_cpu);
> > + spin_unlock(&rq->lock);
> > +
> > + /* The new lock was already acquired in find_lowest */
> > + rq = cpu_rq(new_cpu);
> > + old_state = p->state;
> > + if (!(old_state & state))
> > + goto out;
> > + if (p->se.on_rq)
> > + goto out_running;
> > +
> > + this_cpu = smp_processor_id();
>
> Could we have preempted to get a new this_cpu?
That was a leftover from before we had the double_lock inside the search
function. I will clean this up.
>
> > + cpu = task_cpu(p);
> > + }
> > + }
> > out_activate:
> > #endif /* CONFIG_SMP */
> > update_rq_clock(rq);
> > @@ -2002,30 +2034,13 @@ out_activate:
> > * to find another CPU it can preempt:
> > */
> > if (rt_task(p) && !TASK_PREEMPTS_CURR(p, rq)) {
> > - struct rq *this_rq = cpu_rq(this_cpu);
> > /*
> > - * Special-case: the task on this CPU can be
> > - * preempted. In that case there's no need to
> > - * trigger reschedules on other CPUs, we can
> > - * mark the current task for reschedule.
> > - *
> > - * (Note that it's safe to access this_rq without
> > - * extra locking in this particular case, because
> > - * we are on the current CPU.)
> > + * FIXME: Do we still need to do this here anymore, or
> > + * does the preemption-check above suffice. The path
> > + * that makes my head hurt is when we have the
> > + * task_running->out_activate path
> > */
> > - if (TASK_PREEMPTS_CURR(p, this_rq))
> > - set_tsk_need_resched(this_rq->curr);
> > - else
> > - /*
> > - * Neither the intended target runqueue
> > - * nor the current CPU can take this task.
> > - * Trigger a reschedule on all other CPUs
> > - * nevertheless, maybe one of them can take
> > - * this task:
> > - */
> > - smp_send_reschedule_allbutself_cpumask(p->cpus_allowed);
> > -
> > - schedstat_inc(this_rq, rto_wakeup);
> > + push_rt_tasks(rq);
>
> I think the question is, doesn't this make the above not needed? The
> push_rt_tasks should do what the previous condition did.
>
> Maybe I'm missing something.
Well, only that it has a few efficiency related advantages (1) to doing
this check before the activate() call. You are correct that either
place would yield correct behavior.
(1) -> We can save the overhead of an unnecessary activate/deactivate
cycle, and avoid placing the system (even if only briefly) into overload
which will potentially affect other CPUs if they happen to call
_schedule() during the window.
>
> > } else {
> > /*
> > * Sync wakeups (i.e. those types of wakeups where the waker
> > @@ -2360,13 +2375,12 @@ static inline void finish_task_switch(struct rq *rq, struct task_struct *prev)
> > * the lock was owned by prev, we need to release it
> > * first via finish_lock_switch and then reaquire it.
> > */
> > - if (unlikely(rt_task(current))) {
> > + if (unlikely(rq->rt_nr_running > 1)) {
>
> Heh, I guess that would work.
>
> -- Steve
>
> > spin_lock(&rq->lock);
> > - /* push_rt_task will return true if it moved an RT */
> > - while (push_rt_task(rq))
> > - ;
> > + push_rt_tasks(rq);
> > spin_unlock(&rq->lock);
> > }
> > +
> > #endif
> > fire_sched_in_preempt_notifiers(current);
> > trace_stop_sched_switched(current);
> >
> >
> >
-
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