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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 Mon, 2011-01-03 at 19:05 +0100, Oleg Nesterov wrote:
> On 12/24, Peter Zijlstra wrote:
> >
> > @@ -2680,24 +2681,17 @@ void wake_up_new_task(struct task_struct
> > {
> > unsigned long flags;
> > struct rq *rq;
> > - int cpu __maybe_unused = get_cpu();
>
> Wait, I think this not right.
>
> > #ifdef CONFIG_SMP
> > rq = task_rq_lock(p, &flags);
> > - p->state = TASK_WAKING;
> >
> > /*
> > * Fork balancing, do it here and not earlier because:
> > * - cpus_allowed can change in the fork path
> > * - any previously selected cpu might disappear through hotplug
> > - *
> > - * We set TASK_WAKING so that select_task_rq() can drop rq->lock
> > - * without people poking at ->cpus_allowed.
> > */
> > - cpu = select_task_rq(rq, p, SD_BALANCE_FORK, 0);
> > - set_task_cpu(p, cpu);
> > + set_task_cpu(p, select_task_rq(p, SD_BALANCE_FORK, 0));
> >
> > - p->state = TASK_RUNNING;
> > task_rq_unlock(rq, &flags);
>
> We need preempt_disable() to protect us against CPU hotplug. This task
> was never activated, it won't be found/migrated if that CPU goes away
> before we take task_rq_lock().

Ah, indeed. I've replaced it by keeping IRQs disabled over the whole
function, no need to restore and save them in the middle.


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