From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp1.linuxfoundation.org (smtp1.linux-foundation.org [172.17.192.35]) by mail.linuxfoundation.org (Postfix) with ESMTPS id D43A8FF for ; Wed, 12 Aug 2015 13:38:08 +0000 (UTC) Received: from e37.co.us.ibm.com (e37.co.us.ibm.com [32.97.110.158]) by smtp1.linuxfoundation.org (Postfix) with ESMTPS id 5D00212F for ; Wed, 12 Aug 2015 13:38:08 +0000 (UTC) Received: from /spool/local by e37.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 12 Aug 2015 07:38:07 -0600 Received: from b03cxnp08025.gho.boulder.ibm.com (b03cxnp08025.gho.boulder.ibm.com [9.17.130.17]) by d03dlp01.boulder.ibm.com (Postfix) with ESMTP id E4F6C1FF0046 for ; Wed, 12 Aug 2015 07:29:14 -0600 (MDT) Received: from d03av04.boulder.ibm.com (d03av04.boulder.ibm.com [9.17.195.170]) by b03cxnp08025.gho.boulder.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id t7CDb8ej45678714 for ; Wed, 12 Aug 2015 06:37:08 -0700 Received: from d03av04.boulder.ibm.com (loopback [127.0.0.1]) by d03av04.boulder.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id t7CDc35A028060 for ; Wed, 12 Aug 2015 07:38:05 -0600 Date: Wed, 12 Aug 2015 06:38:02 -0700 From: "Paul E. McKenney" To: Andy Lutomirski Message-ID: <20150812133802.GP3895@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20150811183312.GE3895@linux.vnet.ibm.com> <20150811214729.GH3895@linux.vnet.ibm.com> <20150812005117.GJ3895@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Cc: "ksummit-discuss@lists.linuxfoundation.org" , Peter Zijlstra , "linux-kernel@vger.kernel.org" , Ingo Molnar , Chris Metcalf , Christoph Lameter Subject: Re: [Ksummit-discuss] [BELATED CORE TOPIC] context tracking / nohz / RCU state List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Tue, Aug 11, 2015 at 06:16:01PM -0700, Andy Lutomirski wrote: > On Tue, Aug 11, 2015 at 5:51 PM, Paul E. McKenney > wrote: > > 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. > > Except that something wants vtime for idle, too, so maybe just > kernel_to_idle(). On the other hand, the idle loop is already fully > stocked with vtime stuff. But vtime can work with approximation, and RCU cannot. Also vtime needs to measure time, and RCU needs to count transitions. So I am having some difficulty seeing the benefit of unifying vtime's and RCU's idle entry/exit mechanism. Now, if you are instead arguing for co-location of these mechanisms, that might well be a different issue. Thanx, Paul