linux-kernel - Re: [PATCH 2/3] sched: enforce per-cpu utilization limits on runtime balancing

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Date:	Wed, 3 Mar 2010 17:59:26 +0100
From:	Fabio Checconi <fchecconi@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>,
	Dario Faggioli <faggioli@...dalf.sssup.it>,
	Michael Trimarchi <michael@...dence.eu.com>,
	Dhaval Giani <dhaval@...is.sssup.it>,
	Tommaso Cucinotta <t.cucinotta@...up.it>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] sched: enforce per-cpu utilization limits on runtime
 balancing

> From: Peter Zijlstra <peterz@...radead.org>
> Date: Thu, Feb 25, 2010 09:28:22PM +0100
>
> On Tue, 2010-02-23 at 19:56 +0100, Fabio Checconi wrote:
> > +               /*
> > +                * Make sure that before restarting balancing everyone
> > +                * sees the new free bandwidth.
> > +                */
> > +               smp_wmb(); 
> 
> You cannot order something on its own, barriers come in pairs, where is
> his?

It was coupled with the acquire barrier implied by the spin_lock(rt_b)
in the balance path, as the other wmb() before starting the reset (more
details on the reply about this last barrier, as per your comment they
can be removed).
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