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    SubjectRe: [PATCH] sched: correct how RT task is picked
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    On Tue, May 10, 2011 at 9:04 PM, Hillf Danton <dhillf@gmail.com> wrote:
    > When picking RT task for given CPU,
    > [1] if the cpu is invalid for cpumask test, right result could not be

    'cpu is invalid' means weather we care it or not, it's not real 'invalid'

    > 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,

    No, cpumask_test_cpu(cpu, &p->cpus_allowed) doesn't mean
    p->rt.nr_cpus_allowed > 1.

    Thanks,
    Yong

    > 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@gmail.com>
    > ---
    >
    > --- 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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