From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2528AECAAA3 for ; Fri, 26 Aug 2022 01:42:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244556AbiHZBmd (ORCPT ); Thu, 25 Aug 2022 21:42:33 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48182 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231253AbiHZBma (ORCPT ); Thu, 25 Aug 2022 21:42:30 -0400 Received: from szxga03-in.huawei.com (szxga03-in.huawei.com [45.249.212.189]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3A09DB3B27; Thu, 25 Aug 2022 18:42:29 -0700 (PDT) Received: from dggpemm500024.china.huawei.com (unknown [172.30.72.57]) by szxga03-in.huawei.com (SkyGuard) with ESMTP id 4MDMws2Q1tzGpqw; Fri, 26 Aug 2022 09:40:45 +0800 (CST) Received: from dggpemm500006.china.huawei.com (7.185.36.236) by dggpemm500024.china.huawei.com (7.185.36.203) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Fri, 26 Aug 2022 09:42:13 +0800 Received: from [10.174.178.55] (10.174.178.55) by dggpemm500006.china.huawei.com (7.185.36.236) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Fri, 26 Aug 2022 09:42:12 +0800 Subject: Re: [PATCH v5 1/1] rcu: Simplify the code logic of rcu_init_nohz() To: CC: Frederic Weisbecker , Neeraj Upadhyay , Josh Triplett , "Steven Rostedt" , Mathieu Desnoyers , Lai Jiangshan , Joel Fernandes , , References: <20220825092311.179-1-thunder.leizhen@huawei.com> <20220825092311.179-2-thunder.leizhen@huawei.com> <20220825172654.GP6159@paulmck-ThinkPad-P17-Gen-1> From: "Leizhen (ThunderTown)" Message-ID: <43370a3c-8c7a-c7c7-ac2b-bec2bf6c8728@huawei.com> Date: Fri, 26 Aug 2022 09:42:11 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <20220825172654.GP6159@paulmck-ThinkPad-P17-Gen-1> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.178.55] X-ClientProxiedBy: dggems705-chm.china.huawei.com (10.3.19.182) To dggpemm500006.china.huawei.com (7.185.36.236) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022/8/26 1:26, Paul E. McKenney wrote: > On Thu, Aug 25, 2022 at 05:23:11PM +0800, Zhen Lei wrote: >> When CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or CONFIG_NO_HZ_FULL=y, additional >> CPUs need to be added to 'rcu_nocb_mask'. But 'rcu_nocb_mask' may be not >> available now, due to 'rcu_nocbs' is not specified. Check and initialize >> 'rcu_nocb_mask' before using it. This code simplification strictly follows >> this logic, compared with old implementations, unnecessary crossovers are >> avoided and easy to understand. >> >> Signed-off-by: Zhen Lei > > Much nicer, thank you! > > As usual, I could not resist the urge to wordsmith. Could you please > check to make sure that I did not mess anything up? Okay, thank you very much. The boot option name "rcu_nocb_mask=" is incorrect, should be "rcu_nocbs=". > > Thanx, Paul > > ------------------------------------------------------------------------ > > commit 4ac3b3d1a19943b1522c0b1d0895aefbb80ec294 > Author: Zhen Lei > Date: Thu Aug 25 17:23:11 2022 +0800 > > rcu: Simplify rcu_init_nohz() cpumask handling > > In kernels built with either CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or > CONFIG_NO_HZ_FULL=y, additional CPUs must be added to rcu_nocb_mask. > Except that kernels booted without the rcu_nocb_mask= will not have rcu_nocb_mask= ----> rcu_nocbs= > allocated rcu_nocb_mask. And the current rcu_init_nohz() function uses > its need_rcu_nocb_mask and offload_all local variables to track the > rcu_nocb and nohz_full state. > > But there is a much simpler approach, namely creating a cpumask pointer > to track the default and then using cpumask_available() to check the > rcu_nocb_mask state. This commit takes this approach, thereby simplifying > and shortening the rcu_init_nohz() function. > > Signed-off-by: Zhen Lei > Signed-off-by: Paul E. McKenney > > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h > index 0a5f0ef414845..c8167be2288fa 100644 > --- a/kernel/rcu/tree_nocb.h > +++ b/kernel/rcu/tree_nocb.h > @@ -1210,45 +1210,31 @@ EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload); > void __init rcu_init_nohz(void) > { > int cpu; > - bool need_rcu_nocb_mask = false; > - bool offload_all = false; > struct rcu_data *rdp; > + const struct cpumask *cpumask = NULL; > > #if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL) > - if (!rcu_state.nocb_is_setup) { > - need_rcu_nocb_mask = true; > - offload_all = true; > - } > -#endif /* #if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL) */ > - > -#if defined(CONFIG_NO_HZ_FULL) > - if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask)) { > - need_rcu_nocb_mask = true; > - offload_all = false; /* NO_HZ_FULL has its own mask. */ > - } > -#endif /* #if defined(CONFIG_NO_HZ_FULL) */ > + cpumask = cpu_possible_mask; > +#elif defined(CONFIG_NO_HZ_FULL) > + if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask)) > + cpumask = tick_nohz_full_mask; > +#endif > > - if (need_rcu_nocb_mask) { > + if (cpumask) { > if (!cpumask_available(rcu_nocb_mask)) { > if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) { > pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n"); > return; > } > } > + > + cpumask_or(rcu_nocb_mask, rcu_nocb_mask, cpumask); > rcu_state.nocb_is_setup = true; > } > > if (!rcu_state.nocb_is_setup) > return; > > -#if defined(CONFIG_NO_HZ_FULL) > - if (tick_nohz_full_running) > - cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask); > -#endif /* #if defined(CONFIG_NO_HZ_FULL) */ > - > - if (offload_all) > - cpumask_setall(rcu_nocb_mask); > - > if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) { > pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n"); > cpumask_and(rcu_nocb_mask, cpu_possible_mask, > . > -- Regards, Zhen Lei