[PATCH RFC tip/core/rcu 08/14] rcu/nocb: Reduce __call_rcu_nocb_wake() leaf rcu_node ->lock contention

[Paul E. McKenney]

Currently, __call_rcu_nocb_wake() advances callbacks each time that it
detects excessive numbers of callbacks, 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 and advancing
has not yet been done during this jiffy.

Note that this decision does not take the presence of new callbacks
into account.  That is because on this code path, there will always be
at least one new callback, namely the one we just enqueued.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
---
 kernel/rcu/tree_plugin.h | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 02739366ef5d..efd7f6fa2790 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1869,6 +1869,8 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
 				 unsigned long flags)
 				 __releases(rdp->nocb_lock)
 {
+	unsigned long cur_gp_seq;
+	unsigned long j;
 	long len;
 	struct task_struct *t;
 
@@ -1897,12 +1899,17 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
 	} else if (len > rdp->qlen_last_fqs_check + qhimark) {
 		/* ... or if many callbacks queued. */
 		rdp->qlen_last_fqs_check = len;
-		if (rdp->nocb_cb_sleep ||
-		    !rcu_segcblist_ready_cbs(&rdp->cblist)) {
+		j = jiffies;
+		if (j != rdp->nocb_gp_adv_time &&
+		    rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
+		    rcu_seq_done(&rdp->mynode->gp_seq, cur_gp_seq)) {
 			rcu_advance_cbs_nowake(rdp->mynode, rdp);
+			rdp->nocb_gp_adv_time = j;
+		}
+		if (rdp->nocb_cb_sleep ||
+		    !rcu_segcblist_ready_cbs(&rdp->cblist))
 			wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE,
 					   TPS("WakeOvfIsDeferred"));
-		}
 		rcu_nocb_unlock_irqrestore(rdp, flags);
 	} else {
 		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot"));
-- 
2.17.1