From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754910Ab3GASQO (ORCPT ); Mon, 1 Jul 2013 14:16:14 -0400 Received: from relay3-d.mail.gandi.net ([217.70.183.195]:56706 "EHLO relay3-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754538Ab3GASQN (ORCPT ); Mon, 1 Jul 2013 14:16:13 -0400 X-Originating-IP: 173.246.103.110 Date: Mon, 1 Jul 2013 11:16:01 -0700 From: Josh Triplett To: "Paul E. McKenney" Cc: 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, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu Subject: Re: [PATCH RFC nohz_full v2 2/7] nohz_full: Add rcu_dyntick data for scalable detection of all-idle state Message-ID: <20130701181601.GA7964@jtriplet-mobl1> References: <20130628200949.GA17458@linux.vnet.ibm.com> <1372450222-19420-1-git-send-email-paulmck@linux.vnet.ibm.com> <1372450222-19420-2-git-send-email-paulmck@linux.vnet.ibm.com> <20130701153150.GB2923@leaf> <20130701155220.GL3773@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130701155220.GL3773@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 On Mon, Jul 01, 2013 at 08:52:20AM -0700, Paul E. McKenney wrote: > On Mon, Jul 01, 2013 at 08:31:50AM -0700, Josh Triplett wrote: > > On Fri, Jun 28, 2013 at 01:10:17PM -0700, Paul E. McKenney wrote: > > > From: "Paul E. McKenney" > > > > > > This commit adds fields to the rcu_dyntick structure that are used to > > > detect idle CPUs. These new fields differ from the existing ones in > > > that the existing ones consider a CPU executing in user mode to be idle, > > > where the new ones consider CPUs executing in user mode to be busy. > > > > Can you explain, both in the commit messages and in the comments added > > by the next commit, *why* this code doesn't consider userspace a > > quiescent state? > > Good point! Does the following explain it? > > Although one of RCU's quiescent states is usermode execution, > it is not a full-system idle state. This is because the purpose > of the full-system idle state is not RCU, but rather determining > when accurate timekeeping can safely be disabled. Whenever > accurate timekeeping is required in a CONFIG_NO_HZ_FULL kernel, > at least one CPU must keep the scheduling-clock tick going. > If even one CPU is executing in user mode, accurate timekeeping > is requires, particularly for architectures where gettimeofday() > and friends do not enter the kernel. Only when all CPUs are > really and truly idle can accurate timekeeping be disabled, > allowing all CPUs to turn off the scheduling clock interrupt, > thus greatly improving energy efficiency. > > This naturally raises the question "Why is this code in RCU rather > than in timekeeping?", and the answer is that RCU has the data > and infrastructure to efficiently make this determination. Good explanation, thanks. This also naturally raises the question "How can we let userspace get accurate time without forcing a timer tick?". - Josh Triplett