Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933338AbbHLAvY (ORCPT ); Tue, 11 Aug 2015 20:51:24 -0400 Received: from e33.co.us.ibm.com ([32.97.110.151]:51744 "EHLO e33.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932485AbbHLAvW (ORCPT ); Tue, 11 Aug 2015 20:51:22 -0400 X-Helo: d03dlp02.boulder.ibm.com X-MailFrom: paulmck@linux.vnet.ibm.com X-RcptTo: linux-kernel@vger.kernel.org Date: Tue, 11 Aug 2015 17:51:17 -0700 From: "Paul E. McKenney" To: Andy Lutomirski Cc: "ksummit-discuss@lists.linuxfoundation.org" , "linux-kernel@vger.kernel.org" , Christoph Lameter , =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker , Ingo Molnar , Thomas Gleixner , "H. Peter Anvin" , Peter Zijlstra , Chris Metcalf , Rik van Riel Subject: Re: [BELATED CORE TOPIC] context tracking / nohz / RCU state Message-ID: <20150812005117.GJ3895@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20150811183312.GE3895@linux.vnet.ibm.com> <20150811214729.GH3895@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15081200-0009-0000-0000-00000D2D07C8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3106 Lines: 62 On Tue, Aug 11, 2015 at 02:52:59PM -0700, Andy Lutomirski wrote: > On Tue, Aug 11, 2015 at 2:47 PM, Paul E. McKenney > wrote: > > On Tue, Aug 11, 2015 at 12:07:54PM -0700, Andy Lutomirski wrote: > >> On Tue, Aug 11, 2015 at 11:33 AM, Paul E. McKenney > >> wrote: > >> > On Tue, Aug 11, 2015 at 10:49:36AM -0700, Andy Lutomirski wrote: > >> >> This is a bit late, but here goes anyway. > >> >> > >> >> Having played with the x86 context tracking hooks for awhile, I think > >> >> it would be nice if core code that needs to be aware of CPU context > >> >> (kernel, user, idle, guest, etc) could come up with single, > >> >> comprehensible, easily validated set of hooks that arch code is > >> >> supposed to call. > >> >> > >> >> Currently we have: > >> >> > >> >> - RCU hooks, which come in a wide variety to notify about IRQs, NMIs, etc. > >> > > >> > Something about people yelling at me for waking up idle CPUs, thus > >> > degrading their battery lifetimes. ;-) > >> > > >> >> - Context tracking hooks. Only used by some arches. Calling these > >> >> calls the RCU hooks for you in most cases. They have weird > >> >> interactions with interrupts and they're slow. > >> > > >> > Combining these would be good, but there are subtleties. For example, > >> > some arches don't have context tracking, but RCU still needs to correctly > >> > identify idle CPUs without in any way interrupting or awakening that CPU. > >> > It would be good to make this faster, but it does have to work. > >> > >> Could we maybe have one set of old RCU-only (no context tracking) > >> callbacks and a completely separate set of callbacks for arches that > >> support full context tracking? The implementation of the latter would > >> presumably call into RCU. > > > > It should be possible for RCU to use context tracking if it is available > > and to have RCU maintain its own state otherwise, if that is what you > > are getting at. Assuming that the decision is global and made at either > > build or boot time, anyway. Having some CPUs tracking context and others > > not sounds like an invitation for subtle bugs. > > I think that, if this happens, the decision should be made at build > time, per arch, and not be configurable. If x86_64 uses context > tracking, then I think x86_64 shouldn't need additional RCU callbacks, > assuming that context tracking is comprehensive enough for RCU's > purposes. If by "shouldn't need additional RCU callbacks" you mean that x86_64 shouldn't need to call the existing rcu_user_enter() and rcu_user_exit() functions, I agree. Ditto for rcu_irq_enter(), rcu_irq_exit(), rcu_nmi_enter(), rcu_nmi_exit(), I would guess. But would be necessary to invoke rcu_idle_enter() and rcu_idle_exit(), especially for CONFIG_NO_HZ_FULL_SYSIDLE=y kernels. Thanx, 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/