[PATCH v2] rcu/kfree: Do not request RCU when not needed

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

On ChromeOS, using this with the increased timeout, we see that we almost always
never need to initiate a new grace period. Testing also shows this frees large
amounts of unreclaimed memory, under intense kfree_rcu() pressure.

Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
v1->v2: Same logic but use polled grace periods instead of sampling gp_seq.

 kernel/rcu/tree.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 591187b6352e..ed41243f7a49 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
+ * @gp_snap: The GP snapshot recorded at the last scheduling of monitor work.
  * @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 +2965,7 @@ struct kfree_rcu_cpu {
 	struct kfree_rcu_cpu_work krw_arr[KFREE_N_BATCHES];
 	raw_spinlock_t lock;
 	struct delayed_work monitor_work;
+	unsigned long gp_snap;
 	bool initialized;
 	int count;
 
@@ -3167,6 +3169,7 @@ schedule_delayed_monitor_work(struct kfree_rcu_cpu *krcp)
 			mod_delayed_work(system_wq, &krcp->monitor_work, delay);
 		return;
 	}
+	krcp->gp_snap = get_state_synchronize_rcu();
 	queue_delayed_work(system_wq, &krcp->monitor_work, delay);
 }
 
@@ -3217,7 +3220,10 @@ 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);
+			if (poll_state_synchronize_rcu(krcp->gp_snap))
+				queue_work(system_wq, &krwp->rcu_work.work);
+			else
+				queue_rcu_work(system_wq, &krwp->rcu_work);
 		}
 	}
 
-- 
2.38.1.431.g37b22c650d-goog