From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753020Ab0AJBNA (ORCPT ); Sat, 9 Jan 2010 20:13:00 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751520Ab0AJBNA (ORCPT ); Sat, 9 Jan 2010 20:13:00 -0500 Received: from tomts16-srv.bellnexxia.net ([209.226.175.4]:51352 "EHLO tomts16-srv.bellnexxia.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751179Ab0AJBM7 (ORCPT ); Sat, 9 Jan 2010 20:12:59 -0500 Date: Sat, 9 Jan 2010 20:12:55 -0500 From: Mathieu Desnoyers To: "Paul E. McKenney" Cc: Steven Rostedt , Oleg Nesterov , Peter Zijlstra , linux-kernel@vger.kernel.org, Ingo Molnar , akpm@linux-foundation.org, josh@joshtriplett.org, tglx@linutronix.de, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, laijs@cn.fujitsu.com, dipankar@in.ibm.com Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier Message-ID: <20100110011255.GE25790@Krystal> References: <20100108235338.GA18050@Krystal> <20100109002043.GD6816@linux.vnet.ibm.com> <20100109010231.GA25368@Krystal> <20100109012128.GF6816@linux.vnet.ibm.com> <20100109023842.GA1696@Krystal> <20100109054215.GB9044@linux.vnet.ibm.com> <20100109192006.GA23672@Krystal> <1263078327.28171.3792.camel@gandalf.stny.rr.com> <1263079000.28171.3795.camel@gandalf.stny.rr.com> <20100110000318.GD9044@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline In-Reply-To: <20100110000318.GD9044@linux.vnet.ibm.com> X-Editor: vi X-Info: http://krystal.dyndns.org:8080 X-Operating-System: Linux/2.6.27.31-grsec (i686) X-Uptime: 20:11:55 up 24 days, 9:30, 5 users, load average: 0.00, 0.07, 0.11 User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > On Sat, Jan 09, 2010 at 06:16:40PM -0500, Steven Rostedt wrote: > > On Sat, 2010-01-09 at 18:05 -0500, Steven Rostedt wrote: > > > > > Then we should have O(tasks) for spinlocks taken, and > > > O(min(tasks, CPUS)) for IPIs. > > > > And for nr tasks >> CPUS, this may help too: > > > > > cpumask = 0; > > > foreach task { > > > > if (cpumask == online_cpus) > > break; > > > > > spin_lock(task_rq(task)->rq->lock); > > > if (task_rq(task)->curr == task) > > > cpu_set(task_cpu(task), cpumask); > > > spin_unlock(task_rq(task)->rq->lock); > > > } > > > send_ipi(cpumask); > > Good point, erring on the side of sending too many IPIs is safe. One > might even be able to just send the full set if enough of the CPUs were > running the current process and none of the remainder were running > real-time threads. And yes, it would then be necessary to throttle > calls to sys_membarrier(). > > Quickly hiding behind a suitable boulder... ;-) :) One quick counter-argument against IPI-to-all: that will wake up all CPUs, including those which are asleep. Not really good for energy-saving. Mathieu > > Thanx, Paul -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68