rcu/nocb: Reduce contention at no-CBs registry-time CB advancement

Currently, __call_rcu_nocb_wake() conditionally acquires the leaf rcu_node
structure's ->lock, and only afterwards does rcu_advance_cbs_nowake()
check to see if it is possible to advance callbacks without potentially
needing to awaken the grace-period kthread.  Given that the no-awaken
check can be done locklessly, this commit reverses the order, so that
rcu_advance_cbs_nowake() is invoked without holding the leaf rcu_node
structure's ->lock and rcu_advance_cbs_nowake() checks the grace-period
state before conditionally acquiring that lock, thus reducing the number
of needless acquistions of the leaf rcu_node structure's ->lock.

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

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a6ddfae6978d612369d83a4dffbcaf2ff6462634..ec320658aeef623dec4a179d8ebc6af2610fbb8b 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1341,10 +1341,11 @@ static bool rcu_advance_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
 static void __maybe_unused rcu_advance_cbs_nowake(struct rcu_node *rnp,
                                                  struct rcu_data *rdp)
 {
-       raw_lockdep_assert_held_rcu_node(rnp);
-       if (!rcu_seq_state(rcu_seq_current(&rnp->gp_seq)))
+       if (!rcu_seq_state(rcu_seq_current(&rnp->gp_seq)) ||
+           !raw_spin_trylock_rcu_node(rnp))
                return;
        WARN_ON_ONCE(rcu_advance_cbs(rnp, rdp));
+       raw_spin_unlock_rcu_node(rnp);
 }
 
 /*

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 12212764ecd8d2ae9aa157b20c2bd6a12b20db45..09c15f619e7815b567024f9951e85a0ee1b29eec 100644 (file)
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1668,10 +1668,8 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
                if (!rdp->nocb_cb_sleep &&
                    rcu_segcblist_ready_cbs(&rdp->cblist)) {
                        // Already going full tilt, so don't try to rewake.
-               } else if (rcu_segcblist_pend_cbs(&rdp->cblist) &&
-                          raw_spin_trylock_rcu_node(rdp->mynode)) {
+               } else if (rcu_segcblist_pend_cbs(&rdp->cblist)) {
                        rcu_advance_cbs_nowake(rdp->mynode, rdp);
-                       raw_spin_unlock_rcu_node(rdp->mynode);
                } else {
                        wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE,
                                           TPS("WakeOvfIsDeferred"));