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    SubjectRe: [RFC][PATCH 08/17] sched: Drop the rq argument to sched_class::select_task_rq()
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    On Wed, 2010-12-29 at 22:31 +0800, Yong Zhang wrote:
    > On Fri, Dec 24, 2010 at 01:23:46PM +0100, Peter Zijlstra wrote:
    > > In preparation of calling select_task_rq() without rq->lock held, drop
    > > the dependency on the rq argument.
    > >
    > > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    > > ---
    > > @@ -3416,27 +3409,22 @@ void sched_exec(void)
    > > {
    > > struct task_struct *p = current;
    > > unsigned long flags;
    > > - struct rq *rq;
    > > int dest_cpu;
    > >
    > > - rq = task_rq_lock(p, &flags);
    > > - dest_cpu = p->sched_class->select_task_rq(rq, p, SD_BALANCE_EXEC, 0);
    > > + raw_spin_lock_irqsave(&p->pi_lock, flags);
    >
    > Seems this should go to patch 07/17 ;)

    Ah, the reason its here is that this patch removes the rq argument and
    thus we no longer need rq->lock. So this part relies on the property
    introduced by patch 7.

    > > + dest_cpu = p->sched_class->select_task_rq(p, SD_BALANCE_EXEC, 0);
    > > if (dest_cpu == smp_processor_id())
    > > goto unlock;
    > >
    > > - /*
    > > - * select_task_rq() can race against ->cpus_allowed
    > > - */
    > > - if (cpumask_test_cpu(dest_cpu, &p->cpus_allowed) &&
    > > - likely(cpu_active(dest_cpu)) && migrate_task(p, rq)) {
    > > + if (likely(cpu_active(dest_cpu)) && need_migrate_task(p)) {
    >
    > If we drop rq_lock, need_migrate_task() maybe return true but
    > p is already running on other cpu. Thus we do a wrong migration
    > call.

    Yeah, too bad.. ;-) exec load balancing is more an optimistic thing
    anyway, if it got rebalanced under out feet we don't care.


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