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

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Date:	Thu, 18 Nov 2010 16:32:29 +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

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.

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