From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1163672AbeCBABZ (ORCPT ); Thu, 1 Mar 2018 19:01:25 -0500 Received: from LGEAMRELO13.lge.com ([156.147.23.53]:56909 "EHLO lgeamrelo13.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1163541AbeCBABX (ORCPT ); Thu, 1 Mar 2018 19:01:23 -0500 X-Original-SENDERIP: 156.147.1.151 X-Original-MAILFROM: byungchul.park@lge.com X-Original-SENDERIP: 10.177.222.184 X-Original-MAILFROM: byungchul.park@lge.com Subject: Re: [PATCH] rcu: Clean up rcu_init_nohz() by removing unnecessary statements To: paulmck@linux.vnet.ibm.com Cc: jiangshanlai@gmail.com, josh@joshtriplett.org, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, linux-kernel@vger.kernel.org, kernel-team@lge.com References: <1519808695-28097-1-git-send-email-byungchul.park@lge.com> <20180228184106.GN3777@linux.vnet.ibm.com> From: Byungchul Park Message-ID: <166b5b47-8b6f-4b35-df4c-b42b47a173da@lge.com> Date: Fri, 2 Mar 2018 09:01:19 +0900 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 MIME-Version: 1.0 In-Reply-To: <20180228184106.GN3777@linux.vnet.ibm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 3/1/2018 3:41 AM, Paul E. McKenney wrote: > On Wed, Feb 28, 2018 at 06:04:55PM +0900, Byungchul Park wrote: >> Since the commit 44c65ff2e3b0(rcu: Eliminate NOCBs CPU-state Kconfig >> options) made nocb-cpus identified only through the rcu_nocbs= boot >> parameter, we don't have to care NOCBs CPU-state Kconfig options >> anymore, which means now we can just rely on rcu_nocb_mask to >> decide whether going ahead in rcu_init_nohz(). >> >> Remove the deprecated code. >> >> Signed-off-by: Byungchul Park > > Good catch! However, you missed a (relatively harmless) bug in my commit > 44c65ff2e3b0, namely that if neither the nohz_full= nor the rcu_nocbs= > kernel boot parameters specify any CPUs, we don't need to allocate > rcu_nocb_mask. (That is, when both of those parameters are omitted.) > > Now, if the rcu_nocbs= kernel boot parameter was specified, we know that > rcu_nocb_mask was already allocated in rcu_nocb_setup(). So in > rcu_init_nohz() we only need to do the allocation if NO_HZ_FULL needs > some NOCBs CPUs, that is, when tick_nohz_full_running and when there > is at least one CPU specified in tick_nohz_full_mask. Why didn't I catch it in advance? :) > So the change that is actually needed is to reverse the initialization > of need_rcu_nocb_mask, that is, to initialize it to false rather than > to true. I annotated your patch with my reasoning and have a patch > below with your Reported-by. Please take a look and let me know what > you think. No doubt. I agree with you. Acked-by: Byungchul Park > If I am not too confused, the only effect of this bug was to needlessly > allocate rcu_nocb_mask and to initialize it to all zeros bits, but please > let me know if I missed something. I think so as you. -- Thanks, Byungchul