From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754536Ab2GFDrJ (ORCPT ); Thu, 5 Jul 2012 23:47:09 -0400 Received: from relay4-d.mail.gandi.net ([217.70.183.196]:50492 "EHLO relay4-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752109Ab2GFDrI (ORCPT ); Thu, 5 Jul 2012 23:47:08 -0400 X-Originating-IP: 217.70.178.135 X-Originating-IP: 50.43.46.74 Date: Thu, 5 Jul 2012 20:46:48 -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@efficios.com, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu, patches@linaro.org Subject: Re: [PATCH tip/core/rcu] Make RCU_FAST_NO_HZ respect nohz= boot parameter Message-ID: <20120706034648.GA27047@leaf> References: <20120705223731.GA27608@linux.vnet.ibm.com> <20120705230208.GB26150@leaf> <20120706002944.GL2522@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120706002944.GL2522@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 Thu, Jul 05, 2012 at 05:29:44PM -0700, Paul E. McKenney wrote: > On Thu, Jul 05, 2012 at 04:02:08PM -0700, Josh Triplett wrote: > > On Thu, Jul 05, 2012 at 03:37:31PM -0700, Paul E. McKenney wrote: > > > If the nohz= boot parameter disables nohz, then RCU_FAST_NO_HZ needs to > > > also disable itself. This commit therefore checks for tick_nohz_enabled > > > being zero, disabling rcu_prepare_for_idle() if so. This patch assumes > > > that tick_nohz_enabled can change at runtime: If this is not the case, > > > then a simpler approach suffices. > > > > Allowing nohz to change at runtime seems like an entirely unnecessary > > bit of added complexity. (So does having a boot parameter for it, but > > that one at least seems easier to handle.) > > I will let representatives from the various distros expound to you on > their one-binary-only strategy for kernel builds. ;-) I'm aware. However, the subset of people wanting to turn off nohz seems sufficiently small at this point that I'd *hope* distro kernels could just always have it turned on. :) In any case, as I said, the ability to change it at runtime seems like the primary bit of complexity; the ability to change it at boot time seems straightforward to handle. > > What would the patch look like if you can assume nohz will never change > > at runtime? And does anyone have a use case for changing nohz at > > runtime, rather than at boot time? > > It would be a little bit simpler, but would break in very odd and > difficult-to-debug ways if anyone ever did allow it to change at runtime, > for example, to accommodate systems subject to varying workloads. Granted, but it doesn't seem worth preemtively making RCU more complicated to accomodate a use case that nobody has said they have yet. :) - Josh Triplett