linux-kernel - [PATCH v2 1/2] rcu/kvfree: Use polled API in a slow path

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Message-Id: <20240904143938.119964-2-urezki@gmail.com>
Date: Wed,  4 Sep 2024 16:39:37 +0200
From: "Uladzislau Rezki (Sony)" <urezki@...il.com>
To: "Paul E . McKenney" <paulmck@...nel.org>
Cc: RCU <rcu@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Neeraj upadhyay <Neeraj.Upadhyay@....com>,
	Boqun Feng <boqun.feng@...il.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	Uladzislau Rezki <urezki@...il.com>,
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: [PATCH v2 1/2] rcu/kvfree: Use polled API in a slow path

For a single argument use a polled API to see if a GP is
already passed. This allows to bypass an extra GP request
in a slow path.

Allocating a page or dynamic rcu_head might take some and
still fail, in that scenario a GP which is requested on entry
of kvfree_call_rcu() can be elapsed. Benefit from this.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
---
 kernel/rcu/tree.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index e641cc681901..182772494cb0 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3789,6 +3789,7 @@ add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp,
 void kvfree_call_rcu(struct rcu_head *head, void *ptr)
 {
 	unsigned long flags;
+	struct rcu_gp_oldstate old_snap;
 	struct kfree_rcu_cpu *krcp;
 	bool success;
 
@@ -3799,8 +3800,10 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
 	 * only. For other places please embed an rcu_head to
 	 * your data.
 	 */
-	if (!head)
+	if (!head) {
 		might_sleep();
+		start_poll_synchronize_rcu_full(&old_snap);
+	}
 
 	// Queue the object but don't yet schedule the batch.
 	if (debug_rcu_head_queue(ptr)) {
@@ -3853,7 +3856,7 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
 	 */
 	if (!success) {
 		debug_rcu_head_unqueue((struct rcu_head *) ptr);
-		synchronize_rcu();
+		cond_synchronize_rcu_full(&old_snap);
 		kvfree(ptr);
 	}
 }
-- 
2.39.2