Re: [PATCH] sched: Simplify cpu-hot-unplug task migration

[Oleg Nesterov <oleg@redhat.com>]

Peter, sorry for delay.

I was going to read this patch carefully today, but due to the holiday
in the Czech Republic I have to drink (too much) beer instead ;)

This means you should probably ignore my question, but can't resist...

> -static void migrate_dead_tasks(unsigned int dead_cpu)
> -{
> -	struct rq *rq = cpu_rq(dead_cpu);
> -	struct task_struct *next;
> +	rq->stop = NULL;

(or we could do current->state = TASK_INTERRUPTIPLE, afaics)

>  	for ( ; ; ) {
> -		if (!rq->nr_running)
> +		/*
> +		 * There's this thread running, bail when that's the only
> +		 * remaining thread.
> +		 */
> +		if (rq->nr_running == 1)
>  			break;

I was very much confused, and I was going to say this is wrong.
However, now I think this is correct, just the comment is not
right.

There is another running thread we should not migrate, rq->idle.
If nothing else, dequeue_task_idle() should be never called.

But, if I understand correctly, ->nr_running does not account
the idle thread, and this is what makes this correct.

Correct?

Oleg.