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Subject: Re: [PATCH] sched: Simplify cpu-hot-unplug task migration
From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Raistlin <raistlin@linux.it>, Ingo Molnar <mingo@elte.hu>,
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Date: Wed, 17 Nov 2010 20:42:34 +0100
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On Wed, 2010-11-17 at 20:27 +0100, Oleg Nesterov wrote:
> 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)

Ah, you missed a patch that made pick_next_task_stop() look like:

static struct task_struct *pick_next_task_stop(struct rq *rq)
{
        struct task_struct *stop = rq->stop;

        if (stop && stop->se.on_rq)
                return stop;

        return NULL;
}

> >  	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.

In fact, dequeue_task_idle() will yell if you try that ;-)

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

Right, I can add: (the idle thread is not counted in nr_running), if
that makes things clearer for you; however its a quite fundamental
property, we don't consider the idle task a proper runnable entity, its
simply the thing we do when there's nothing else to do.