rcu: Make rcu_migrate_callbacks wake GP kthread when needed

The rcu_migrate_callbacks() function invokes rcu_advance_cbs()
twice, ignoring the return value.  This is OK at pressent because of
failsafe code that does the wakeup when needed.  However, this failsafe
code acquires the root rcu_node structure's lock frequently, while
rcu_migrate_callbacks() does so only once per CPU-offline operation.

This commit therefore makes rcu_migrate_callbacks()
wake up the RCU GP kthread when either call to rcu_advance_cbs()
returns true, thus removing need for the failsafe code.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>

1 files changed, 6 insertions, 2 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 6ef1f2b4a6d3..f75eb5174021 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3876,6 +3876,7 @@ static void rcu_migrate_callbacks(int cpu, struct rcu_state *rsp)
         struct rcu_data *my_rdp;
         struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
         struct rcu_node *rnp_root = rcu_get_root(rdp->rsp);
+        bool needwake;
 
         if (rcu_is_nocb_cpu(cpu) || rcu_segcblist_empty(&rdp->cblist))
                 return;  /* No callbacks to migrate. */
@@ -3887,12 +3888,15 @@ static void rcu_migrate_callbacks(int cpu, struct rcu_state *rsp)
                 return;
         }
         raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */
-        rcu_advance_cbs(rsp, rnp_root, rdp); /* Leverage recent GPs. */
-        rcu_advance_cbs(rsp, rnp_root, my_rdp); /* Assign GP to pending CBs. */
+        /* Leverage recent GPs and set GP for new callbacks. */
+        needwake = rcu_advance_cbs(rsp, rnp_root, rdp) ||
+                   rcu_advance_cbs(rsp, rnp_root, 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);
+        if (needwake)
+                rcu_gp_kthread_wake(rsp);
         WARN_ONCE(rcu_segcblist_n_cbs(&rdp->cblist) != 0 ||
                   !rcu_segcblist_empty(&rdp->cblist),
                   "rcu_cleanup_dead_cpu: Callbacks on offline CPU %d: qlen=%lu, 1stCB=%p\n",