rcu: Check for NOCB CPUs and empty lists earlier in CB migration

The current CPU-hotplug RCU-callback-migration code checks
for the source (newly offlined) CPU being a NOCBs CPU down in
rcu_send_cbs_to_orphanage().  This commit simplifies callback migration a
bit by moving this check up to rcu_migrate_callbacks().  This commit also
adds a check for the source CPU having no callbacks, which eases analysis
of the rcu_send_cbs_to_orphanage() and rcu_adopt_orphan_cbs() functions.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 3 insertions, 4 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f5acf34247fb..aeea697d6f9f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3862,10 +3862,6 @@ rcu_send_cbs_to_orphanage(int cpu, struct rcu_state *rsp,
 {
         lockdep_assert_held(&rsp->orphan_lock);
 
-        /* No-CBs CPUs do not have orphanable callbacks. */
-        if (!IS_ENABLED(CONFIG_HOTPLUG_CPU) || rcu_is_nocb_cpu(rdp->cpu))
-                return;
-
         /*
          * Orphan the callbacks.  First adjust the counts.  This is safe
          * because _rcu_barrier() excludes CPU-hotplug operations, so it
@@ -3935,6 +3931,9 @@ static void rcu_migrate_callbacks(int cpu, struct rcu_state *rsp)
         struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
         struct rcu_node *rnp = rdp->mynode;  /* Outgoing CPU's rdp & rnp. */
 
+        if (rcu_is_nocb_cpu(cpu) || rcu_segcblist_empty(&rdp->cblist))
+                return;  /* No callbacks to migrate. */
+
         raw_spin_lock_irqsave(&rsp->orphan_lock, flags);
         rcu_send_cbs_to_orphanage(cpu, rsp, rnp, rdp);
         rcu_adopt_orphan_cbs(rsp, flags);