Re: [PATCH v5 1/1] rcu: Simplify the code logic of rcu_init_nohz()

["Paul E. McKenney" <paulmck@kernel.org>]

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 <thunder.leizhen@huawei.com>

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 <thunder.leizhen@huawei.com>
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 <thunder.leizhen@huawei.com>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

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,