[PATCH -next 4/8] rcu/nocb: Add warning if no rcuog wake up attempt happened during overload

[Joel Fernandes <joelagnelf@nvidia.com>]

To be sure we have no rcog wake ups that were lost, add a warning
to cover the case where the rdp is overloaded with callbacks but
no wake up was attempted.

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 kernel/rcu/tree.c      | 4 ++++
 kernel/rcu/tree.h      | 1 +
 kernel/rcu/tree_nocb.h | 6 +++++-
 3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 293bbd9ac3f4..78c045a5ef03 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3767,6 +3767,10 @@ 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);
 	}
+#ifdef CONFIG_RCU_NOCB_CPU
+	if (wake_nocb)
+		rdp->nocb_gp_wake_attempt = true;
+#endif
 	rcu_nocb_unlock(rdp);
 	if (wake_nocb)
 		wake_nocb_gp(rdp, false);
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 653fb4ba5852..74bd6a2a2f84 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -257,6 +257,7 @@ struct rcu_data {
 	unsigned long nocb_gp_loops;	/* # passes through wait code. */
 	struct swait_queue_head nocb_gp_wq; /* For nocb kthreads to sleep on. */
 	bool nocb_cb_sleep;		/* Is the nocb CB thread asleep? */
+	bool nocb_gp_wake_attempt;	/* Was a rcuog wakeup attempted? */
 	struct task_struct *nocb_cb_kthread;
 	struct list_head nocb_head_rdp; /*
 					 * Head of rcu_data list in wakeup chain,
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index daff2756cd90..7e9d465c8ab1 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -546,6 +546,7 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
 	lazy_len = READ_ONCE(rdp->lazy_len);
 	if (was_alldone) {
 		rdp->qlen_last_fqs_check = len;
+		rdp->nocb_gp_wake_attempt = true;
 		rcu_nocb_unlock(rdp);
 		// Only lazy CBs in bypass list
 		if (lazy_len && bypass_len == lazy_len) {
@@ -563,7 +564,8 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
 
 		return;
 	} else if (len > rdp->qlen_last_fqs_check + qhimark) {
-		/* ... or if many callbacks queued. */
+		/* Callback overload condition. */
+		WARN_ON_ONCE(!rdp->nocb_gp_wake_attempt);
 		rdp->qlen_last_fqs_check = len;
 		j = jiffies;
 		if (j != rdp->nocb_gp_adv_time &&
@@ -688,6 +690,7 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
 		     bypass_ncbs > 2 * qhimark)) {
 			flush_bypass = true;
 		} else if (!bypass_ncbs && rcu_segcblist_empty(&rdp->cblist)) {
+			rdp->nocb_gp_wake_attempt = false;
 			rcu_nocb_unlock_irqrestore(rdp, flags);
 			continue; /* No callbacks here, try next. */
 		}
@@ -1254,6 +1257,7 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 			continue;
 		}
 		rcu_nocb_try_flush_bypass(rdp, jiffies);
+		rdp->nocb_gp_wake_attempt = true;
 		rcu_nocb_unlock_irqrestore(rdp, flags);
 		wake_nocb_gp(rdp, false);
 		sc->nr_to_scan -= _count;
-- 
2.34.1