[RFC PATCH 15/18] cgroup/cpuset: Enable RCU NO-CB CPU offloading of newly isolated CPUs

[Waiman Long]

Make use of the provided 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.

Also add a new rcu_nocbs_enabled() helper function to determine if RCU
NO-CB CPU offloading can be done.

As nohz_full can now be specified without any CPU list, drop the test
for cpumask_empty(tick_nohz_full_mask) in rcu_init_nohz().

The RCU NO-CB CPU offloading feature can only used if either "rcs_nocbs"
or the "nohz_full" boot command parameters are used so that the proper
RCU NO-CB resources are properly initialized at boot time.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 include/linux/rcupdate.h |  2 ++
 kernel/cgroup/cpuset.c   | 14 ++++++++++++++
 kernel/rcu/tree_nocb.h   |  7 ++++++-
 3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 120536f4c6eb..642b80a4f071 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -140,6 +140,7 @@ 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);
+bool rcu_nocbs_enabled(void);
 
 #define RCU_NOCB_LOCKDEP_WARN(c, s) RCU_LOCKDEP_WARN(c, s)
 
@@ -149,6 +150,7 @@ 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 bool rcu_nocbs_enabled(void) { return false; }
 
 #define RCU_NOCB_LOCKDEP_WARN(c, s)
 
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 45c82c18bec4..de9cb92a0fc7 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1416,6 +1416,20 @@ static int do_housekeeping_exclude_cpumask(void *arg __maybe_unused)
 	}
 #endif
 
+	if (rcu_nocbs_enabled()) {
+		/*
+		 * Enable RCU NO-CB CPU offloading/deoffloading for the affected CPUs
+		 */
+		for_each_cpu(cpu, isolcpus_update_state.cpus) {
+			if (cpumask_test_cpu(cpu, icpus))
+				ret = rcu_nocb_cpu_offload(cpu);
+			else
+				ret = rcu_nocb_cpu_deoffload(cpu);
+			if (WARN_ON_ONCE(ret))
+				break;
+		}
+	}
+
 	if (icpus != isolated_cpus)
 		kfree(icpus);
 	return ret;
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index e6cd56603cad..4d49a745b871 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1293,7 +1293,7 @@ void __init rcu_init_nohz(void)
 	struct shrinker * __maybe_unused lazy_rcu_shrinker;
 
 #if defined(CONFIG_NO_HZ_FULL)
-	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
+	if (tick_nohz_full_running)
 		cpumask = tick_nohz_full_mask;
 #endif
 
@@ -1365,6 +1365,11 @@ static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
 	mutex_init(&rdp->nocb_gp_kthread_mutex);
 }
 
+bool rcu_nocbs_enabled(void)
+{
+	return !!rcu_state.nocb_is_setup;
+}
+
 /*
  * If the specified CPU is a no-CBs CPU that does not already have its
  * rcuo CB kthread, spawn it.  Additionally, if the rcuo GP kthread
-- 
2.50.0