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[34.150.221.228]) by smtp.gmail.com with ESMTPSA id bw12-20020a05622a098c00b0031eddc83560sm15034809qtb.90.2022.08.25.12.24.06 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 25 Aug 2022 12:24:06 -0700 (PDT) Date: Thu, 25 Aug 2022 19:24:05 +0000 From: Joel Fernandes To: "Paul E. McKenney" Cc: Zhen Lei , Frederic Weisbecker , Neeraj Upadhyay , Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , rcu@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v5 1/1] rcu: Simplify the code logic of rcu_init_nohz() Message-ID: References: <20220825092311.179-1-thunder.leizhen@huawei.com> <20220825092311.179-2-thunder.leizhen@huawei.com> <20220825172654.GP6159@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220825172654.GP6159@paulmck-ThinkPad-P17-Gen-1> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 25, 2022 at 10:26:54AM -0700, 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? > > 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 > 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 Reviewed-by: Joel Fernandes (Google) thanks, - Joel > > 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,