[PATCH] sched: correct how RT task is picked

[Hillf Danton]

When picking RT task for given CPU,
[1] if the cpu is invalid for cpumask test, right result could not be
reached even by further checking nr_cpus_allowed,
on the other hand, the input cpu is valid in two cases that
pick_next_highest_task_rt() is called, thus the invalid input cpu
looks over-concern.
[2] if the cpu is valid for cpumask test, further checking
nr_cpus_allowed looks overwork, since it is computed based on
cpus_allowed,
what is more, the combination of cpus_allowed and nr_cpus_allowed
could lead to incorrect result if the input cpu == rq->cpu, as in the
case of next_prio() where no pulling task is concerned.

In this work, invalid cpu is not removed but leads to negative result,
but nr_cpus_allowed is.

Signed-off-by: Hillf Danton <dhillf@xxxxxxxxx>
---

--- a/kernel/sched_rt.c	2011-04-27 11:48:50.000000000 +0800
+++ b/kernel/sched_rt.c	2011-05-10 20:30:38.000000000 +0800
@@ -1149,10 +1149,12 @@ static void deactivate_task(struct rq *r

 static int pick_rt_task(struct rq *rq, struct task_struct *p, int cpu)
 {
-	if (!task_running(rq, p) &&
-	    (cpu < 0 || cpumask_test_cpu(cpu, &p->cpus_allowed)) &&
-	    (p->rt.nr_cpus_allowed > 1))
-		return 1;
+	if (!task_running(rq, p)) {
+		if (cpu < 0)
+			return 0;
+		if (cpumask_test_cpu(cpu, &p->cpus_allowed))
+			return 1;
+	}
 	return 0;
 }
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