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SubjectRe: [RFC][PATCH 08/17] sched: Drop the rq argument to sched_class::select_task_rq()
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().

Oleg.



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