[PATCH 06/23] rcu/nocbs: Allow runtime changes in RCU NOCBS cpumask

[Waiman Long <longman@redhat.com>]

We can make use of the rcu_nocb_cpu_offload()/rcu_nocb_cpu_deoffload()
APIs to enable RCU NO-CB CPU offloading of newly isolated CPUs and
deoffloading of de-isolated CPUs.

Add a new rcu_nocb_update_cpus() helper to do that and call it directly
from housekeeping_update() when the HK_TYPE_KERNEL_NOISE cpumask is
being changed.

This dynamic RCU NO-CB CPU offloading feature can only used if either
"rcs_nocbs" or the "nohz_full" boot command parameters are used with or
without parameter so that the proper RCU NO-CB resources are properly
initialized at boot time.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 include/linux/rcupdate.h |  2 ++
 kernel/rcu/tree_nocb.h   | 22 ++++++++++++++++++++++
 kernel/sched/isolation.c |  4 +++-
 3 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 04f3f86a4145..987e3d1d413e 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -150,6 +150,7 @@ void rcu_init_nohz(void);
 int rcu_nocb_cpu_offload(int cpu);
 int rcu_nocb_cpu_deoffload(int cpu);
 void rcu_nocb_flush_deferred_wakeup(void);
+void rcu_nocb_update_cpus(struct cpumask *cpumask);
 
 #define RCU_NOCB_LOCKDEP_WARN(c, s) RCU_LOCKDEP_WARN(c, s)
 
@@ -159,6 +160,7 @@ static inline void rcu_init_nohz(void) { }
 static inline int rcu_nocb_cpu_offload(int cpu) { return -EINVAL; }
 static inline int rcu_nocb_cpu_deoffload(int cpu) { return 0; }
 static inline void rcu_nocb_flush_deferred_wakeup(void) { }
+static inline void rcu_nocb_update_cpus(struct cpumask *cpumask) { }
 
 #define RCU_NOCB_LOCKDEP_WARN(c, s)
 
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 2d06dcb61f37..b2daba1e5cb9 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1173,6 +1173,28 @@ int rcu_nocb_cpu_offload(int cpu)
 }
 EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
 
+void rcu_nocb_update_cpus(struct cpumask *cpumask)
+{
+	int cpu, ret;
+
+	if (!rcu_state.nocb_is_setup) {
+		pr_warn_once("Dynamic RCU NOCB cannot be enabled without nohz_full/rcu_nocbs kernel boot parameter!\n");
+		return;
+	}
+
+	for_each_cpu_andnot(cpu, cpumask, rcu_nocb_mask) {
+		ret = rcu_nocb_cpu_offload(cpu);
+		if (WARN_ON_ONCE(ret))
+			return;
+	}
+
+	for_each_cpu_andnot(cpu, rcu_nocb_mask, cpumask) {
+		ret = rcu_nocb_cpu_deoffload(cpu);
+		if (WARN_ON_ONCE(ret))
+			return;
+	}
+}
+
 #ifdef CONFIG_RCU_LAZY
 static unsigned long
 lazy_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index 48b155e0b290..b5635484ec69 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -181,8 +181,10 @@ int housekeeping_update(struct cpumask *isol_mask, unsigned long flags)
 	if ((housekeeping.flags & flags) != flags)
 		WRITE_ONCE(housekeeping.flags, housekeeping.flags | flags);
 
-	if (flags & HK_FLAG_KERNEL_NOISE)
+	if (flags & HK_FLAG_KERNEL_NOISE) {
 		tick_nohz_full_update_cpus(isol_mask);
+		rcu_nocb_update_cpus(isol_mask);
+	}
 
 	synchronize_rcu();
 
-- 
2.53.0