[PATCH v7 01/11] rcu: Wake up nocb gp thread on rcu_barrier_entrain()

["Joel Fernandes (Google)" <joel@joelfernandes.org>]

From: Frederic Weisbecker <frederic@kernel.org>

In preparation of RCU lazy changes, wake up the RCU nocb gp thread if
needed after an entrain. Otherwise, the RCU barrier callback can wait in
the queue for several seconds before the lazy callbacks in front of it
are serviced.

Reported-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 kernel/rcu/tree.c      | 11 +++++++++++
 kernel/rcu/tree_nocb.h |  4 ++++
 2 files changed, 15 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 5ec97e3f7468..04f33191e5ed 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3894,6 +3894,8 @@ static void rcu_barrier_entrain(struct rcu_data *rdp)
 {
 	unsigned long gseq = READ_ONCE(rcu_state.barrier_sequence);
 	unsigned long lseq = READ_ONCE(rdp->barrier_seq_snap);
+	bool wake_nocb = false;
+	bool was_done = false;
 
 	lockdep_assert_held(&rcu_state.barrier_lock);
 	if (rcu_seq_state(lseq) || !rcu_seq_state(gseq) || rcu_seq_ctr(lseq) != rcu_seq_ctr(gseq))
@@ -3902,6 +3904,7 @@ static void rcu_barrier_entrain(struct rcu_data *rdp)
 	rdp->barrier_head.func = rcu_barrier_callback;
 	debug_rcu_head_queue(&rdp->barrier_head);
 	rcu_nocb_lock(rdp);
+	was_done = rcu_rdp_is_offloaded(rdp) && !rcu_segcblist_pend_cbs(&rdp->cblist);
 	WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies));
 	if (rcu_segcblist_entrain(&rdp->cblist, &rdp->barrier_head)) {
 		atomic_inc(&rcu_state.barrier_cpu_count);
@@ -3909,7 +3912,15 @@ static void rcu_barrier_entrain(struct rcu_data *rdp)
 		debug_rcu_head_unqueue(&rdp->barrier_head);
 		rcu_barrier_trace(TPS("IRQNQ"), -1, rcu_state.barrier_sequence);
 	}
+
+	/*
+	 * If bypass list was non-empty, wake up the nocb GP thread otherwise
+	 * bypass/lazy CBs may not be noticed, and can cause real long delays!
+	 */
+	wake_nocb = was_done && rcu_segcblist_pend_cbs(&rdp->cblist);
 	rcu_nocb_unlock(rdp);
+	if (wake_nocb)
+		wake_nocb_gp(rdp, false);
 	smp_store_release(&rdp->barrier_seq_snap, gseq);
 }
 
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index f77a6d7e1356..6caade0683dd 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1558,6 +1558,10 @@ static void rcu_init_one_nocb(struct rcu_node *rnp)
 {
 }
 
+static bool wake_nocb_gp(struct rcu_data *rdp, bool force)
+{
+}
+
 static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
 				  unsigned long j)
 {
-- 
2.38.0.rc1.362.ged0d419d3c-goog