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

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Message-ID: <20101117192713.GA11910@redhat.com>
Date:	Wed, 17 Nov 2010 20:27:13 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Raistlin <raistlin@...ux.it>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Chris Friesen <cfriesen@...tel.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Darren Hart <darren@...art.com>,
	Johan Eker <johan.eker@...csson.com>,
	"p.faure" <p.faure@...tech.ch>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Claudio Scordino <claudio@...dence.eu.com>,
	michael trimarchi <trimarchi@...is.sssup.it>,
	Fabio Checconi <fabio@...dalf.sssup.it>,
	Tommaso Cucinotta <cucinotta@...up.it>,
	Juri Lelli <juri.lelli@...il.com>,
	Nicola Manica <nicola.manica@...i.unitn.it>,
	Luca Abeni <luca.abeni@...tn.it>,
	Dhaval Giani <dhaval@...is.sssup.it>,
	Harald Gustafsson <hgu1972@...il.com>,
	paulmck <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] sched: Simplify cpu-hot-unplug task migration

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.

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