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 37658C32772 for ; Tue, 23 Aug 2022 15:09:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243376AbiHWPJt (ORCPT ); Tue, 23 Aug 2022 11:09:49 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36174 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S243309AbiHWPJb (ORCPT ); Tue, 23 Aug 2022 11:09:31 -0400 Received: from szxga02-in.huawei.com (szxga02-in.huawei.com [45.249.212.188]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 891F4107758; Tue, 23 Aug 2022 05:39:15 -0700 (PDT) Received: from dggpemm500022.china.huawei.com (unknown [172.30.72.54]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4MBnlC3xpmzlVpd; Tue, 23 Aug 2022 19:56:55 +0800 (CST) Received: from dggpemm500006.china.huawei.com (7.185.36.236) by dggpemm500022.china.huawei.com (7.185.36.162) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Tue, 23 Aug 2022 20:00:08 +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; Tue, 23 Aug 2022 20:00:07 +0800 Subject: Re: [PATCH v4 1/2] rcu: Eliminate rcu_state.nocb_is_setup To: CC: Frederic Weisbecker , Neeraj Upadhyay , Josh Triplett , "Steven Rostedt" , Mathieu Desnoyers , Lai Jiangshan , Joel Fernandes , , References: <20220817014253.1982-1-thunder.leizhen@huawei.com> <20220817014253.1982-2-thunder.leizhen@huawei.com> <20220822163411.GF6159@paulmck-ThinkPad-P17-Gen-1> From: "Leizhen (ThunderTown)" Message-ID: <89bd14ea-6980-4857-9e2a-9d89c94e88a7@huawei.com> Date: Tue, 23 Aug 2022 20:00:07 +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: <20220822163411.GF6159@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: dggems703-chm.china.huawei.com (10.3.19.180) 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 2022/8/23 0:34, Paul E. McKenney wrote: > On Wed, Aug 17, 2022 at 09:42:52AM +0800, Zhen Lei wrote: >> 'rcu_state.nocb_is_setup' is initialized to true only if 'rcu_nocb_mask' >> successfully allocates memory. So it can be replaced by >> 'cpumask_available(rcu_nocb_mask)'. More importantly, the latter is more >> intuitive, and it has been used in several places. >> >> Signed-off-by: Zhen Lei > > One of the implementations of cpumask_available() does indeed check > for NULL. But here is the other one: > > static inline bool cpumask_available(cpumask_var_t mask) > { > return true; > } Thanks for the heads-up. > > So I have to ask... In a kernel built with CONFIG_CPUMASK_OFFSTACK=n, > will this change really work? Yes, I run the test cases on arm64, which does not turn on option CONFIG_CPUMASK_OFFSTACK by default. I've listed a combination of build options in the cover-letter. In this case, cpumask_empty(rcu_nocb_mask) is true. ---- CONFIG_NO_HZ_FULL=n, CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=n, cmdline without rcu_nocbs [ 0.000000] rcu: Offload RCU callbacks from CPUs: (none). ---- > > Another important question is "Do all of the existing uses of > cpumask_available() really work?" Yes, I do believe that they do The only functional change caused by this patch is: For clarity, CONFIG_RCU_NOCB_CPU=y and CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=n are omitted from the following conditions. When CONFIG_NO_HZ_FULL=n and boot cmdline without 'rcu_nocbs='. or CONFIG_NO_HZ_FULL=y and boot cmdline without 'nohz_full='. The rdp->nocb_gp_kthread and rdp->nocb_cb_kthread threads are still created. But we have provided EXPORT_SYMBOL_GPL functions rcu_nocb_cpu_deoffload() and rcu_nocb_cpu_offload(), which can dynamically modify 'rcu_nocb_mask'. So it seems appropriate to prepare these threads in advance. Of course, it looks like only 'rcutorture' currently uses these two functions now. Otherwise, we can do some optimization: If none of the CPUs in a 'nocb_gp' group is marked in rcu_nocb_mask, this grouping does not need to create corresponding threads "rcuog/%d" and "rcuo%c/%d". And in rcu_init_nohz(): - if (!rcu_state.nocb_is_setup) + if (!cpumask_available(rcu_nocb_mask) || cpumask_empty(rcu_nocb_mask)) return; > work, but it would be good to get another set of eyes on that code. > "All software developers are blind!" ;-) > > Thanx, Paul > >> --- >> kernel/rcu/tree.h | 1 - >> kernel/rcu/tree_nocb.h | 8 +++----- >> 2 files changed, 3 insertions(+), 6 deletions(-) >> >> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h >> index d4a97e40ea9c3e2..06f659c63d2d192 100644 >> --- a/kernel/rcu/tree.h >> +++ b/kernel/rcu/tree.h >> @@ -375,7 +375,6 @@ struct rcu_state { >> arch_spinlock_t ofl_lock ____cacheline_internodealigned_in_smp; >> /* Synchronize offline with */ >> /* GP pre-initialization. */ >> - int nocb_is_setup; /* nocb is setup from boot */ >> }; >> >> /* Values for rcu_state structure's gp_flags field. */ >> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h >> index 0a5f0ef41484518..ff763e7dc53551f 100644 >> --- a/kernel/rcu/tree_nocb.h >> +++ b/kernel/rcu/tree_nocb.h >> @@ -69,7 +69,6 @@ static int __init rcu_nocb_setup(char *str) >> cpumask_setall(rcu_nocb_mask); >> } >> } >> - rcu_state.nocb_is_setup = true; >> return 1; >> } >> __setup("rcu_nocbs", rcu_nocb_setup); >> @@ -1215,7 +1214,7 @@ void __init rcu_init_nohz(void) >> struct rcu_data *rdp; >> >> #if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL) >> - if (!rcu_state.nocb_is_setup) { >> + if (!cpumask_available(rcu_nocb_mask)) { >> need_rcu_nocb_mask = true; >> offload_all = true; >> } >> @@ -1235,10 +1234,9 @@ void __init rcu_init_nohz(void) >> return; >> } >> } >> - rcu_state.nocb_is_setup = true; >> } >> >> - if (!rcu_state.nocb_is_setup) >> + if (!cpumask_available(rcu_nocb_mask)) >> return; >> >> #if defined(CONFIG_NO_HZ_FULL) >> @@ -1299,7 +1297,7 @@ static void rcu_spawn_cpu_nocb_kthread(int cpu) >> struct task_struct *t; >> struct sched_param sp; >> >> - if (!rcu_scheduler_fully_active || !rcu_state.nocb_is_setup) >> + if (!rcu_scheduler_fully_active || !cpumask_available(rcu_nocb_mask)) >> return; >> >> /* If there already is an rcuo kthread, then nothing to do. */ >> -- >> 2.25.1 >> > . > -- Regards, Zhen Lei