From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH v5 7/8] mm: Only IPI CPUs to drain local pages if they exist Date: Sat, 7 Jan 2012 09:05:06 -0800 Message-ID: <20120107170506.GA29261@linux.vnet.ibm.com> References: <1325499859-2262-1-git-send-email-gilad@benyossef.com> <1325499859-2262-8-git-send-email-gilad@benyossef.com> <4F033EC9.4050909@gmail.com> <20120105142017.GA27881@csn.ul.ie> <20120105144011.GU11810@n2100.arm.linux.org.uk> <20120105161739.GD27881@csn.ul.ie> <20120107165201.GA23939@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Russell King - ARM Linux , KOSAKI Motohiro , Gilad Ben-Yossef , linux-kernel@vger.kernel.org, Chris Metcalf , Peter Zijlstra , Frederic Weisbecker , linux-mm@kvack.org, Pekka Enberg , Matt Mackall , Sasha Levin , Rik van Riel , Andi Kleen , Andrew Morton , Alexander Viro , linux-fsdevel@vger.kernel.org, Avi Kivity To: Mel Gorman Return-path: Content-Disposition: inline In-Reply-To: <20120107165201.GA23939@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org On Sat, Jan 07, 2012 at 08:52:01AM -0800, Paul E. McKenney wrote: > 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(). Of course, another trick is to hold the CPU-hotplug lock across the IPI, but this is quite a bit more heavy-weight than disabling preemption. Thanx, Paul -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org