Messages in this thread | | | Date | Sun, 21 Oct 2007 13:59:47 +0200 | From | "Dmitry Adamushko" <> | Subject | Re: [patch 6/8] pull RT tasks |
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On 19/10/2007, Steven Rostedt <rostedt@goodmis.org> wrote:
> [ ... ] > > @@ -2927,6 +2927,13 @@ static void idle_balance(int this_cpu, s > int pulled_task = -1; > unsigned long next_balance = jiffies + HZ; > > + /* > + * pull_rt_task returns true if the run queue changed. > + * But this does not mean we have a task to run. > + */ > + if (unlikely(pull_rt_task(this_rq)) && this_rq->nr_running) > + return; > + > for_each_domain(this_cpu, sd) { > unsigned long interval; > > @@ -3614,6 +3621,7 @@ need_resched_nonpreemptible: > if (unlikely(!rq->nr_running)) > idle_balance(cpu, rq); > > + schedule_balance_rt(rq, prev);
we do pull_rt_task() in idle_balance() so, I think, there is no need to do it twice. i.e. if (unlikely(!rq->nr_running)) idle_balance(cpu, rq); + else + schedule_balance_rt(rq, prev);
hum?
moreover (continuing my previous idea on "don't pull more than 1 task at once"), I wonder whether you really see cases when more than 1 task have been successfully _pushed_ over to other run-queues at once...
I'd expect the push/pull algorithm to naturally avoid such a possibility. Let's say we have a few RT tasks on our run-queue that are currently runnable (but not running)... the question is 'why do they still here?'
(1) because the previous attempt to _push_ them failed; (2) because they were not _pulled_ from other run-queues.
both cases should mean that other run-queues have tasks with higher prios running at the moment.
yes, there is a tiny window in schedule() between deactivate_task() [ which can make this run-queue to look like we can push over to it ] and idle_balance() -> pull_rt_task() _or_ schedule_balance_rt() -> pull_rt_task() [ where this run-queue will try to pull tasks on its own ]
_but_ the run-queue is locked in this case so we wait in double_lock_balance() (from push_rt_task()) and run into the competition with 'src_rq' (which is currently in the 'tiny window' as described above trying to run pull_rt_task()) for getting both self_rq and src_rq locks...
this way, push_rt_task() always knows the task to be pushed (so it can be a bit optimized) --- as it's either a newly woken up RT task with (p->prio > rq->curr->prio) _or_ a preempted RT task (so we know 'task' for both cases).
To sum it up, I think that the pull/push algorithm should be able to naturally accomplish the proper job pushing/pulling 1 task at once (as described above)... any additional actions are just overhead or there is some problem with the algorithm (ah well, or with my understanding :-/ )
-- Best regards, Dmitry Adamushko - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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