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Date: Wed, 17 Nov 2010 20:27:13 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Raistlin <raistlin@linux.it>, Ingo Molnar <mingo@elte.hu>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
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        Darren Hart <darren@dvhart.com>, Johan Eker <johan.eker@ericsson.com>,
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        linux-kernel <linux-kernel@vger.kernel.org>,
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        Fabio Checconi <fabio@gandalf.sssup.it>,
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        paulmck <paulmck@linux.vnet.ibm.com>
Subject: Re: [PATCH] sched: Simplify cpu-hot-unplug task migration
Message-ID: <20101117192713.GA11910@redhat.com>
References: <1289486096.2084.123.camel@laptop> <20101111152750.GA2510@redhat.com> <1289490202.2084.140.camel@laptop> <20101111163242.GA5697@redhat.com> <1289673355.2109.79.camel@laptop> <20101113195857.GA11411@redhat.com> <1289680291.2109.244.camel@laptop> <1289681497.2109.270.camel@laptop> <1289691081.2109.452.camel@laptop> <1289851597.2109.547.camel@laptop>
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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.