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Date: Thu, 18 Nov 2010 16:32:29 +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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        linux-kernel <linux-kernel@vger.kernel.org>,
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        Fabio Checconi <fabio@gandalf.sssup.it>,
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        Juri Lelli <juri.lelli@gmail.com>,
        Nicola Manica <nicola.manica@disi.unitn.it>,
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        paulmck <paulmck@linux.vnet.ibm.com>
Subject: Re: [PATCH] sched: Simplify cpu-hot-unplug task migration
Message-ID: <20101118153229.GA32495@redhat.com>
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On 11/18, Peter Zijlstra wrote:
>
> There is no firm guarantee the dying cpu actually got to running the
> idle thread (there's a guarantee it will at some point), so we ought to
> maintain that wait-loop, possibly using cpu_relax(), I don't see the
> point in calling yield() here.

Agreed. But do we need to wait at all?

With or without this change, even if we know that rq->idle is running
we can't know if it (say) already started play_dead_common() or not.

We are going to call __cpu_die(), afaics it should do the necessary
synchronization in any case.

For example, native_cpu_die() waits for cpu_state == CPU_DEAD in a
loop. Of course it should work in practice (it also does msleep),
but in theory there is no guarantee.

So. Can't we just remove this wait-loop? We know that rq->idle
will be scheduled "soon", I don't understand why it is necessary
to ensure that context_switch() has already happened.

Oleg.