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_nocbs= 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>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Acked-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 0a5f0ef41484518575591f7811eff31d45e751ff..ce526cc2791ca6cc15561d519298c331952cb759 100644 (file)
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1210,45 +1210,33 @@ 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;
-
-#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) */
+       const struct cpumask *cpumask = NULL;
 
 #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) */
+       if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
+               cpumask = tick_nohz_full_mask;
+#endif
+
+       if (IS_ENABLED(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL) &&
+           !rcu_state.nocb_is_setup && !cpumask)
+               cpumask = cpu_possible_mask;
 
-       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,