Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753456Ab3EPR54 (ORCPT ); Thu, 16 May 2013 13:57:56 -0400 Received: from 173-166-109-252-newengland.hfc.comcastbusiness.net ([173.166.109.252]:60607 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751643Ab3EPR5x (ORCPT ); Thu, 16 May 2013 13:57:53 -0400 Date: Thu, 16 May 2013 19:56:02 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: Steven Rostedt , Don Zickus , Frederic Weisbecker , Ingo Molnar , LKML , Andrew Morton , Thomas Gleixner , "H. Peter Anvin" Subject: Re: [PATCH 1/2] nohz: Disable LOCKUP_DETECTOR when NO_HZ_FULL is enabled Message-ID: <20130516175602.GL19669@dyad.programming.kicks-ass.net> References: <1368547372-21011-1-git-send-email-fweisbec@gmail.com> <1368547372-21011-2-git-send-email-fweisbec@gmail.com> <20130515083729.GC10510@laptop.programming.kicks-ass.net> <20130515150652.GP23604@redhat.com> <1368631622.6828.69.camel@gandalf.local.home> <20130515165915.GE13916@laptop.home> <1368637441.6828.70.camel@gandalf.local.home> <20130516081027.GD19669@dyad.programming.kicks-ass.net> <20130516150706.GY4442@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130516150706.GY4442@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1856 Lines: 41 On Thu, May 16, 2013 at 08:07:06AM -0700, Paul E. McKenney wrote: > On Thu, May 16, 2013 at 10:10:27AM +0200, Peter Zijlstra wrote: > > On Wed, May 15, 2013 at 01:04:01PM -0400, Steven Rostedt wrote: > > > On Wed, 2013-05-15 at 18:59 +0200, Peter Zijlstra wrote: > > > > > > > At which point we could run the watchdog without perf_event_task_tick(). > > > > > > At which point we can drop the disable LOCKUP_DETECTOR when NO_HZ_FULL > > > is enabled ;-) > > > > > > > Can we? The thing I'm worried about is RCU (of course!). ISTR we rely on RCU > > working in NMI context. AFAIR for RCU to work, we need to come out of out magic > > NO_HZ state since that would've put RCU into EQS. > > > > Frederic, PaulMck? > > Not sure I understand the question, but hopefully the verbiage below helps. > > Only RCU read-side critical sections need to work in NMI context, > and RCU hooks into nmi_enter() and nmi_exit() to handle this, and this > will work in NO_HZ_FULL in the same way that it works for NO_HZ_IDLE. > > But if there are no NMIs, RCU doesn't care. In other words, RCU needs > to know about NMIs so that it can deal with any RCU read-side critical > sections in the NMI handlers, but RCU doesn't rely on NMIs happening at > any particular time or frequency. I suppose the fundamental question was: will receiving NMIs negate NO_HZ_FULL's functionality? That is, will the getting of NMIs make us drop out of NO_HZ_FULL and re-enable all sorts of things? Because clearly RCU needs to exit from EQS, which might (or might not) mean leaving NO_HZ_FULL. I'm not entirely up-to-date on those details. -- 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/