From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: [PATCH v5 7/8] mm: Only IPI CPUs to drain local pages if they
 exist
Date: Sat, 7 Jan 2012 08:52:01 -0800
Message-ID: <20120107165201.GA23939@linux.vnet.ibm.com>
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Cc: Russell King - ARM Linux <linux@arm.linux.org.uk>,
	KOSAKI Motohiro <kosaki.motohiro@gmail.com>,
	Gilad Ben-Yossef <gilad@benyossef.com>,
	linux-kernel@vger.kernel.org, Chris Metcalf <cmetcalf@tilera.com>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Frederic Weisbecker <fweisbec@gmail.com>, linux-mm@kvack.org,
	Pekka Enberg <penberg@kernel.org>,
	Matt Mackall <mpm@selenic.com>,
	Sasha Levin <levinsasha928@gmail.com>,
	Rik van Riel <riel@redhat.com>,
	Andi Kleen <andi@firstfloor.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-fsdevel@vger.kernel.org, Avi Kivity <avi@redhat.com>
To: Mel Gorman <mel@csn.ul.ie>
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On Thu, Jan 05, 2012 at 04:17:39PM +0000, Mel Gorman wrote:
> On Thu, Jan 05, 2012 at 02:40:11PM +0000, Russell King - ARM Linux wrote:
> > On Thu, Jan 05, 2012 at 02:20:17PM +0000, Mel Gorman wrote:

[ . . . ]

> > I've been chasing that patch and getting no replies what so
> > ever from folk like Peter, Thomas and Ingo.
> > 
> > The problem affects all IPI-raising functions, which mask with
> > cpu_online_mask directly.
> 
> Actually, in one sense I'm glad to hear it because from my brief
> poking around, I was having trouble understanding why we were always
> safe from sending IPIs to CPUs in the process of being offlined.

The trick is to disable preemption (not interrupts!) across the IPI, which
prevents CPU-hotplug's stop_machine() from running.  You also have to
have checked that the CPU is online within this same preemption-disabled
section of code.  This means that the outgoing CPU has to accept IPIs
even after its CPU_DOWN_PREPARE notifier has been called -- right up
to the stop_machine() call to take_cpu_down().

							Thanx, Paul