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SubjectRe: [sched/rt] Optimization of function pull_rt_task()
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On Wed, 2012-04-18 at 14:32 -0400, Steven Rostedt wrote:
> On Mon, 2012-04-16 at 12:06 -0400, Steven Rostedt wrote:
> > On Sun, 2012-04-15 at 23:45 +0400, Kirill Tkhai wrote:
> > > The condition (src_rq->rt.rt_nr_running) is weak because it doesn't
> > > consider the cases when src_rq has only processes bound to it (when
> > > single cpu is allowed). It may be running kernel thread like
> > > migration/x etc.
> > >
> > > So it's better to use more stronger condition which is able to exclude
> > > above conditions. The function has_pushable_tasks() complitely does
> > > this. A task may be pullable for another cpu rq only if he is pushable
> > > for his own queue.
> >
> > I considered this before, and for some reason I never did the change.
> > I'll have to think about it. It seems like this would be the obvious
> > case, but I think there was something not so obvious that caused issues.
> > But I don't remember what it was.
> >
> > I'll have to rethink this again.
>
> I can't find anything wrong with this change. Maybe things change, or I
> was thinking of another change.
>
> I'll apply it and start running my tests against it.

Not only does this seem to work fine, I took it one step further :-)

Peter, do you see anything wrong with this patch?

-- Steve

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 61e3086..b44fd1b 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1416,39 +1416,15 @@ static int pick_rt_task(struct rq *rq, struct task_struct *p, int cpu)
/* Return the second highest RT task, NULL otherwise */
static struct task_struct *pick_next_highest_task_rt(struct rq *rq, int cpu)
{
- struct task_struct *next = NULL;
- struct sched_rt_entity *rt_se;
- struct rt_prio_array *array;
- struct rt_rq *rt_rq;
- int idx;
+ struct plist_head *head = &rq->rt.pushable_tasks;
+ struct task_struct *next;

- for_each_leaf_rt_rq(rt_rq, rq) {
- array = &rt_rq->active;
- idx = sched_find_first_bit(array->bitmap);
-next_idx:
- if (idx >= MAX_RT_PRIO)
- continue;
- if (next && next->prio <= idx)
- continue;
- list_for_each_entry(rt_se, array->queue + idx, run_list) {
- struct task_struct *p;
-
- if (!rt_entity_is_task(rt_se))
- continue;
-
- p = rt_task_of(rt_se);
- if (pick_rt_task(rq, p, cpu)) {
- next = p;
- break;
- }
- }
- if (!next) {
- idx = find_next_bit(array->bitmap, MAX_RT_PRIO, idx+1);
- goto next_idx;
- }
+ plist_for_each_entry(next, head, pushable_tasks) {
+ if (pick_rt_task(rq, next, cpu))
+ return next;
}

- return next;
+ return NULL;
}

static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask);



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