[PATCH RFC] rcu/kfree: Do not request RCU when not needed

["Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx>]

On ChromeOS, I am (almost) always seeing the optimization trigger.
Tested boot up and trace_printk'ing how often it triggers.

Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
 kernel/rcu/tree.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 591187b6352e..3e4c50b9fd33 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2935,6 +2935,7 @@ struct kfree_rcu_cpu_work {
 
 /**
  * struct kfree_rcu_cpu - batch up kfree_rcu() requests for RCU grace period
+ * @rdp: The rdp of the CPU that this kfree_rcu corresponds to.
  * @head: List of kfree_rcu() objects not yet waiting for a grace period
  * @bkvhead: Bulk-List of kvfree_rcu() objects not yet waiting for a grace period
  * @krw_arr: Array of batches of kfree_rcu() objects waiting for a grace period
@@ -2964,6 +2965,8 @@ struct kfree_rcu_cpu {
 	struct kfree_rcu_cpu_work krw_arr[KFREE_N_BATCHES];
 	raw_spinlock_t lock;
 	struct delayed_work monitor_work;
+	struct rcu_data *rdp;
+	unsigned long last_gp_seq;
 	bool initialized;
 	int count;
 
@@ -3167,6 +3170,7 @@ schedule_delayed_monitor_work(struct kfree_rcu_cpu *krcp)
 			mod_delayed_work(system_wq, &krcp->monitor_work, delay);
 		return;
 	}
+	krcp->last_gp_seq = krcp->rdp->gp_seq;
 	queue_delayed_work(system_wq, &krcp->monitor_work, delay);
 }
 
@@ -3217,7 +3221,17 @@ static void kfree_rcu_monitor(struct work_struct *work)
 			// be that the work is in the pending state when
 			// channels have been detached following by each
 			// other.
-			queue_rcu_work(system_wq, &krwp->rcu_work);
+			//
+			// NOTE about gp_seq wrap: In case of gp_seq overflow,
+			// it is possible for rdp->gp_seq to be less than
+			// krcp->last_gp_seq even though a GP might be over. In
+			// this rare case, we would just have one extra GP.
+			if (krcp->last_gp_seq &&
+			    rcu_seq_completed_gp(krcp->last_gp_seq, krcp->rdp->gp_seq)) {
+				queue_work(system_wq, &krwp->rcu_work.work);
+			} else {
+				queue_rcu_work(system_wq, &krwp->rcu_work);
+			}
 		}
 	}
 
@@ -4802,6 +4816,8 @@ static void __init kfree_rcu_batch_init(void)
 	for_each_possible_cpu(cpu) {
 		struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
 
+		krcp->rdp = per_cpu_ptr(&rcu_data, cpu);
+		krcp->last_gp_seq = 0;
 		for (i = 0; i < KFREE_N_BATCHES; i++) {
 			INIT_RCU_WORK(&krcp->krw_arr[i].rcu_work, kfree_rcu_work);
 			krcp->krw_arr[i].krcp = krcp;
-- 
2.38.1.273.g43a17bfeac-goog