[PATCH RFC tip/core/rcu 07/14] rcu/nocb: Reduce nocb_cb_wait() leaf rcu_node ->lock contention

[Paul E. McKenney]

Currently, nocb_cb_wait() advances callbacks on each pass through its
loop, though only if it succeeds in conditionally acquiring its leaf
rcu_node structure's ->lock.  Despite the conditional acquisition of
->lock, this does increase contention.  This commit therefore avoids
advancing callbacks unless there are callbacks in ->cblist whose grace
period has completed.

Note that nocb_cb_wait() doesn't worry about callbacks that have not
yet been assigned a grace period.  The idea is that the only reason for
nocb_cb_wait() to advance callbacks is to allow it to continue invoking
callbacks.  Time will tell whether this is the correct choice.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
---
 kernel/rcu/tree_plugin.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 5db39ce1cae1..02739366ef5d 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2076,6 +2076,7 @@ static int rcu_nocb_gp_kthread(void *arg)
  */
 static void nocb_cb_wait(struct rcu_data *rdp)
 {
+	unsigned long cur_gp_seq;
 	unsigned long flags;
 	bool needwake_gp = false;
 	struct rcu_node *rnp = rdp->mynode;
@@ -2088,7 +2089,9 @@ static void nocb_cb_wait(struct rcu_data *rdp)
 	local_bh_enable();
 	lockdep_assert_irqs_enabled();
 	rcu_nocb_lock_irqsave(rdp, flags);
-	if (raw_spin_trylock_rcu_node(rnp)) { /* irqs already disabled. */
+	if (rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
+	    rcu_seq_done(&rnp->gp_seq, cur_gp_seq) &&
+	    raw_spin_trylock_rcu_node(rnp)) { /* irqs already disabled. */
 		needwake_gp = rcu_advance_cbs(rdp->mynode, rdp);
 		raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
 	}
-- 
2.17.1