From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759217AbbFBMg1 (ORCPT ); Tue, 2 Jun 2015 08:36:27 -0400 Received: from e31.co.us.ibm.com ([32.97.110.149]:53689 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759037AbbFBMgT (ORCPT ); Tue, 2 Jun 2015 08:36:19 -0400 Date: Tue, 2 Jun 2015 05:36:13 -0700 From: "Paul E. McKenney" To: Ingo Molnar Cc: linux-kernel@vger.kernel.org, will.deacon@arm.com, milos@redhat.com, joe@perches.com, pdaly@codeaurora.org, nicolas.iooss_linux@m4x.org, hofrat@osadl.org, ying.xue@windriver.com, paul.gortmaker@windriver.com, bobby.prani@gmail.com, alexey.kodanev@oracle.com Subject: Re: [GIT PULL rcu/next] RCU commits for 4.2 Message-ID: <20150602123612.GE5989@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20150531044830.GA3176@linux.vnet.ibm.com> <20150602063153.GA31128@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150602063153.GA31128@gmail.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15060212-8236-0000-0000-00000BFEFBEA Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 02, 2015 at 08:31:53AM +0200, Ingo Molnar wrote: > * Paul E. McKenney wrote: > > > Hello, Ingo, > > > > The changes in this series include the following: > > > > 1. Remove all uses of RCU-protected array indexes. These were posted > > to LKML at https://lkml.org/lkml/2015/5/12/827. > > > > 2. Documentation updates. These were posted to LKML at > > https://lkml.org/lkml/2015/5/12/848. > > > > 3. Miscellaneous fixes. These were posted to LKML at > > https://lkml.org/lkml/2015/5/12/876. > > > > 4. CPU-hotplug updates. These were posted to LKML at > > https://lkml.org/lkml/2015/5/12/880. > > > > 5. Initialization/Kconfig updates. These were posted to LKML at > > https://lkml.org/lkml/2015/5/12/918, however, the updates to > > Tree RCU's initialization have been deferred to 4.3. > > > > 6. Updates to Tiny RCU. These were posted to LKML at > > https://lkml.org/lkml/2015/5/12/957. > > > > 7. Torture-testing updates. These were posted to LKML at > > https://lkml.org/lkml/2015/5/12/964. [ . . . ] > Pulled, thanks a lot Paul! > > I am wondering about the following small detail. You have reworked the whole RCU > Kconfig machinery with the introduction of RCU_EXPERT and smart selection of > defaults. (very nice!) > > There's a single interactive option left when a user configures RCU 'anew' and > keeps CONFIG_RCU_EXPERT disabled: > > Offload RCU callback processing from boot-selected CPUs (RCU_NOCB_CPU) [N/y/?] (NEW) > > I'm wondering whether we could put this behind CONFIG_RCU_EXPERT as well, and > disable it by default? > > The argument for this is very simple IMHO: NO_HZ_FULL depends on nocb_cpu support, > and actually selects it, so there's no way to misconfigure it. > > Without NO_HZ_FULL it might still make sense to use nocb_cpu, but it's very much > an advanced, expert option. > > With this change we could make RCU essentially configuration free in the common > case! :-) And the -rt folks could presumably add a select clause as well, where needed. And I cannot recall a specific reason for not doing so. I have queued the following patch, so let's see how it goes. ;-) Thanx, Paul ------------------------------------------------------------------------ rcu: Hide RCU_NOCB_CPU behind RCU_EXPERT This commit prevents Kconfig from asking the user about RCU_NOCB_CPU unless the user really wants to be asked. Reported-by: Ingo Molnar Signed-off-by: Paul E. McKenney Cc: Steven Rostedt Cc: Sebastian Andrzej Siewior Cc: Thomas Gleixner diff --git a/init/Kconfig b/init/Kconfig index 5b8726c10685..67a0156fa091 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -697,6 +697,7 @@ config RCU_BOOST_DELAY config RCU_NOCB_CPU bool "Offload RCU callback processing from boot-selected CPUs" depends on TREE_RCU || PREEMPT_RCU + depends on RCU_EXPERT default n help Use this option to reduce OS jitter for aggressive HPC or