Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754129Ab3DOCEJ (ORCPT ); Sun, 14 Apr 2013 22:04:09 -0400 Received: from ozlabs.org ([203.10.76.45]:48522 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752839Ab3DOCEI (ORCPT ); Sun, 14 Apr 2013 22:04:08 -0400 Date: Mon, 15 Apr 2013 12:03:54 +1000 From: Paul Mackerras To: "Paul E. McKenney" Cc: Josh Triplett , linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu Subject: Re: [PATCH tip/core/rcu 6/7] rcu: Drive quiescent-state-forcing delay from HZ Message-ID: <20130415020354.GB3401@iris.ozlabs.ibm.com> References: <20130412231846.GA20038@linux.vnet.ibm.com> <1365808754-20762-1-git-send-email-paulmck@linux.vnet.ibm.com> <1365808754-20762-6-git-send-email-paulmck@linux.vnet.ibm.com> <20130412235401.GA8140@jtriplet-mobl1> <20130413063804.GV29861@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130413063804.GV29861@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2378 Lines: 46 On Fri, Apr 12, 2013 at 11:38:04PM -0700, Paul E. McKenney wrote: > On Fri, Apr 12, 2013 at 04:54:02PM -0700, Josh Triplett wrote: > > On Fri, Apr 12, 2013 at 04:19:13PM -0700, Paul E. McKenney wrote: > > > From: "Paul E. McKenney" > > > > > > Systems with HZ=100 can have slow bootup times due to the default > > > three-jiffy delays between quiescent-state forcing attempts. This > > > commit therefore auto-tunes the RCU_JIFFIES_TILL_FORCE_QS value based > > > on the value of HZ. However, this would break very large systems that > > > require more time between quiescent-state forcing attempts. This > > > commit therefore also ups the default delay by one jiffy for each > > > 256 CPUs that might be on the system (based off of nr_cpu_ids at > > > runtime, -not- NR_CPUS at build time). > > > > > > Reported-by: Paul Mackerras > > > Signed-off-by: Paul E. McKenney > > > > Something seems very wrong if RCU regularly hits the fqs code during > > boot; feels like there's some more straightforward solution we're > > missing. What causes these CPUs to fall under RCU's scrutiny during > > boot yet not actually hit the RCU codepaths naturally? > > The problem is that they are running HZ=100, so that RCU will often > take 30-60 milliseconds per grace period. At that point, you only > need 16-30 grace periods to chew up a full second, so it is not all > that hard to eat up the additional 8-12 seconds of boot time that > they were seeing. IIRC, UP boot was costing them 4 seconds. I added some instrumentation, which counted 202 calls to synchronize_sched() during boot (Fedora 17 minimal install + development tools) with a 3.8.0 kernel on a 4-cpu KVM virtual machine on a POWER7. Without this patch, those 202 calls take up a total of 4.32 seconds; with it, they take up 3.6 seconds. The kernel is compiled with HZ=100 and NR_CPUS=1024, like the standard Fedora kernel. I suspect a lot of the calls are in udevd and related processes. Interestingly there were no calls to synchronize_rcu_bh or synchronize_sched_expedited. Paul. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/