From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758969Ab3BGQsW (ORCPT ); Thu, 7 Feb 2013 11:48:22 -0500 Received: from e7.ny.us.ibm.com ([32.97.182.137]:40920 "EHLO e7.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758830Ab3BGQsU (ORCPT ); Thu, 7 Feb 2013 11:48:20 -0500 Date: Thu, 7 Feb 2013 08:30:04 -0800 From: "Paul E. McKenney" To: Steven Rostedt Cc: Ingo Molnar , Frederic Weisbecker , LKML , Alessio Igor Bogani , Andrew Morton , Chris Metcalf , Christoph Lameter , Geoff Levand , Gilad Ben Yossef , Hakan Akkan , Li Zhong , Namhyung Kim , Paul Gortmaker , Peter Zijlstra , Thomas Gleixner Subject: Re: [ANNOUNCE] 3.8-rc6-nohz4 Message-ID: <20130207163004.GS2632@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1360175338-6735-1-git-send-email-fweisbec@gmail.com> <1360205415.2621.60.camel@gandalf.local.home> <20130207111025.GD8945@gmail.com> <1360253520.2621.66.camel@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1360253520.2621.66.camel@gandalf.local.home> User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13020716-5806-0000-0000-00001F4FB4D8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 07, 2013 at 11:12:00AM -0500, Steven Rostedt wrote: > On Thu, 2013-02-07 at 12:10 +0100, Ingo Molnar wrote: > > * Steven Rostedt wrote: > > > > > I'll reply to this as I come up with comments. > > > > > > First thing is, don't call it NO_HZ_FULL. A better name would > > > be NO_HZ_CPU. I would like to reserve NO_HZ_FULL when we > > > totally remove jiffies :-) > > > > I don't think we want yet another config option named in a > > weird way. > > > > What we want instead is to just split NO_HZ up into its > > conceptual parts: > > > > CONFIG_NO_HZ_IDLE > > CONFIG_NO_HZ_USER_SPACE > > CONFIG_NO_HZ_KERNEL_SPACE > > > > Where the current status quo is NO_HZ_IDLE=y, and Frederic is > > about to introduce NO_HZ_USER_SPACE=y. When jiffies get removed > > we get NO_HZ_KERNEL_SPACE=y. > > Saying NO_HZ_USER_SPACE is a bit of a misnomer. As we don't just stop > the tick for user space, but it may remained stopped when entering the > kernel. The rule is that when there's just a single task on a CPU, the > tick can stop (no scheduling work needed). But if the task triggers > something that may require a tick (like printk) then the tick will start > again. But just going into the kernel does not designate a tick restart. > > Maybe a better name would be NO_HZ_SINGLE_TASK ? > > > > > The 'CONFIG_NO_HZ' meta-option, which we should leave for easy > > configurability and for compatibility, should get us the > > currently recommended default, which for the time being might > > be: > > > > CONFIG_NO_HZ_IDLE=y > > # CONFIG_NO_HZ_USER_SPACE is disabled > > > > Btw., you could add CONFIG_NO_HZ_KERNEL_SPACE right away, just > > keep it false all the time. That would document our future plans > > pretty well. > > Maybe the removal of jiffies would be NO_HZ_COMPLETE? I suspect that removal of jiffies from the kernel will take a few stages, with RCU being one of the laggards for awhile. Making RCU's state machine depend wholly on process-based execution will take some care and experimentation, especially for extreme and corner-case workloads. For example, having RCU OOM the system just because a specific CPU was unable to run some RCU kthread for an extended time is something to be avoided. ;-) Thanx, Paul