rcu/nocb: Make rcutree_migrate_callbacks() start at leaf rcu_node structure

Because rcutree_migrate_callbacks() is invoked infrequently and because
an exact snapshot of the grace-period state might save some callbacks a
second trip through a grace period, this function has used the root
rcu_node structure.  However, this safe-second-trip optimization
happens only if rcutree_migrate_callbacks() races with grace-period
initialization, so it is not worth the added mental load.  This commit
therefore makes rcutree_migrate_callbacks() start with the leaf rcu_node
structures, as is done elsewhere.

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

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 969ba29..ea479d8 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3157,8 +3157,8 @@ void rcutree_migrate_callbacks(int cpu)
 {
        unsigned long flags;
        struct rcu_data *my_rdp;
+       struct rcu_node *my_rnp;
        struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
-       struct rcu_node *rnp_root = rcu_get_root();
        bool needwake;
 
        if (rcu_segcblist_is_offloaded(&rdp->cblist) ||
@@ -3167,18 +3167,19 @@ void rcutree_migrate_callbacks(int cpu)
 
        local_irq_save(flags);
        my_rdp = this_cpu_ptr(&rcu_data);
+       my_rnp = my_rdp->mynode;
        if (rcu_nocb_adopt_orphan_cbs(my_rdp, rdp, flags)) {
                local_irq_restore(flags);
                return;
        }
-       raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */
+       raw_spin_lock_rcu_node(my_rnp); /* irqs already disabled. */
        /* Leverage recent GPs and set GP for new callbacks. */
-       needwake = rcu_advance_cbs(rnp_root, rdp) ||
-                  rcu_advance_cbs(rnp_root, my_rdp);
+       needwake = rcu_advance_cbs(my_rnp, rdp) ||
+                  rcu_advance_cbs(my_rnp, my_rdp);
        rcu_segcblist_merge(&my_rdp->cblist, &rdp->cblist);
        WARN_ON_ONCE(rcu_segcblist_empty(&my_rdp->cblist) !=
                     !rcu_segcblist_n_cbs(&my_rdp->cblist));
-       raw_spin_unlock_irqrestore_rcu_node(rnp_root, flags);
+       raw_spin_unlock_irqrestore_rcu_node(my_rnp, flags);
        if (needwake)
                rcu_gp_kthread_wake();
        WARN_ONCE(rcu_segcblist_n_cbs(&rdp->cblist) != 0 ||