[RFC PATCH 3/8] rcu/no_cb: Add rcu_nocb_enabled() to expose the rcu_nocb state

[Waiman Long <longman@redhat.com>]

Add a new rcu_nocb_enabled() helper to expose the rcu_nocb state
to other kernel subsystems like cpuset.  That will allow cpuset to
determine if RCU no-callback can be enabled on isolated CPUs within
isolated partitions. If so, the corresponding RCU functions can be
called to enable it when full CPU isolation is requested.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 include/linux/rcupdate.h |  6 ++++++
 kernel/rcu/tree_nocb.h   | 12 ++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index b649344075d2..976d55a3e523 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -120,6 +120,12 @@ void rcu_init(void);
 extern int rcu_scheduler_active;
 void rcu_sched_clock_irq(int user);
 
+#ifdef CONFIG_RCU_NOCB_CPU
+int rcu_nocb_enabled(struct cpumask *out_mask);
+#else
+static inline int rcu_nocb_enabled(struct cpumask *out_mask) { return 0; }
+#endif
+
 #ifdef CONFIG_TASKS_RCU_GENERIC
 void rcu_init_tasks_generic(void);
 #else
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index bbcf6f4152a3..020a347ccd52 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -81,6 +81,18 @@ static int __init parse_rcu_nocb_poll(char *arg)
 }
 __setup("rcu_nocb_poll", parse_rcu_nocb_poll);
 
+/*
+ * Return the rcu_nocb state & optionally copy out rcu_nocb_mask.
+ */
+int rcu_nocb_enabled(struct cpumask *out_mask)
+{
+	if (!rcu_state.nocb_is_setup)
+		return 0;
+	if (out_mask)
+		cpumask_copy(out_mask, rcu_nocb_mask);
+	return 1;
+}
+
 /*
  * Don't bother bypassing ->cblist if the call_rcu() rate is low.
  * After all, the main point of bypassing is to avoid lock contention
-- 
2.39.3