Re: [RFC][PATCH 18/18] sched: Sort hotplug vs ttwu queueing

[Oleg Nesterov]

On 01/04, Peter Zijlstra wrote:
>
> +#ifdef CONFIG_HOTPLUG_CPU
> +static void ttwu_queue_unplug(struct rq *rq)
> +{
> +	struct task_struct *p, *list = xchg(&rq->wake_list, NULL);
> +	unsigned long flags;
> +	int cpu;
> +
> +	if (!list)
> +		return;
> +
> +	while (list) {
> +		p = list;
> +		list = list->wake_entry;
> +
> +		raw_spin_lock_irqsave(&p->pi_lock, flags);
> +		cpu = select_task_rq(p, SD_BALANCE_WAKE, 0);
> +		set_task_cpu(p, cpu);
> +		ttwu_queue(p, cpu);
> +		raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> +	}
> +}
> +#endif
> +
>  /**
>   * try_to_wake_up - wake up a thread
>   * @p: the thread to be awakened
> @@ -6151,6 +6174,11 @@ migration_call(struct notifier_block *nf
>  		migrate_nr_uninterruptible(rq);
>  		calc_global_load_remove(rq);
>  		break;
> +
> +	case CPU_DEAD:
> +		ttwu_queue_unplug(cpu_rq(cpu));

I think this is not strictly needed...

Afaics, take_cpu_down() can simply call sched_ttwu_pending() at the
start. This will activate the pending tasks on the (almost) dead
cpu but we don't care, they will be migrated later.

When __stop_machine(take_cpu_down) returns nobody can use this CPU
as a target, iow rq->wake_list can't be used again.

Oleg.

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