netdev - [PATCH net-next v3 3/3] xsk: remove spin lock protection of cached

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Message-Id: <20251128134601.54678-4-kerneljasonxing@gmail.com>
Date: Fri, 28 Nov 2025 21:46:01 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	bjorn@...nel.org,
	magnus.karlsson@...el.com,
	maciej.fijalkowski@...el.com,
	jonathan.lemon@...il.com,
	sdf@...ichev.me,
	ast@...nel.org,
	daniel@...earbox.net,
	hawk@...nel.org,
	john.fastabend@...il.com,
	horms@...nel.org,
	andrew+netdev@...n.ch
Cc: bpf@...r.kernel.org,
	netdev@...r.kernel.org,
	Jason Xing <kernelxing@...cent.com>
Subject: [PATCH net-next v3 3/3] xsk: remove spin lock protection of cached_prod

From: Jason Xing <kernelxing@...cent.com>

Remove the spin lock protection along with some functions adjusted.

Now cached_prod is fully converted to atomic, which does help in the
contended case where umem is shared between xsks.

Removing that lock can avoid manipulating one extra cacheline in the
extremely hot path which directly improves the performance by around
5% over different platforms as Paolo found[1].

[1]: https://lore.kernel.org/all/4c645223-8c52-40d3-889b-f3cf7fa09f89@redhat.com/

Signed-off-by: Jason Xing <kernelxing@...cent.com>
---
 include/net/xsk_buff_pool.h |  5 -----
 net/xdp/xsk.c               | 21 ++++-----------------
 net/xdp/xsk_buff_pool.c     |  1 -
 3 files changed, 4 insertions(+), 23 deletions(-)

diff --git a/include/net/xsk_buff_pool.h b/include/net/xsk_buff_pool.h
index 92a2358c6ce3..0b1abdb99c9e 100644
--- a/include/net/xsk_buff_pool.h
+++ b/include/net/xsk_buff_pool.h
@@ -90,11 +90,6 @@ struct xsk_buff_pool {
 	 * destructor callback.
 	 */
 	spinlock_t cq_prod_lock;
-	/* Mutual exclusion of the completion ring in the SKB mode.
-	 * Protect: when sockets share a single cq when the same netdev
-	 * and queue id is shared.
-	 */
-	spinlock_t cq_cached_prod_lock;
 	struct xdp_buff_xsk *free_heads[];
 };
 
diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index b63409b1422e..ae8a92c168b8 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -546,17 +546,6 @@ static int xsk_wakeup(struct xdp_sock *xs, u8 flags)
 	return dev->netdev_ops->ndo_xsk_wakeup(dev, xs->queue_id, flags);
 }
 
-static int xsk_cq_reserve_locked(struct xsk_buff_pool *pool)
-{
-	int ret;
-
-	spin_lock(&pool->cq_cached_prod_lock);
-	ret = xsk_cq_cached_prod_reserve(pool->cq);
-	spin_unlock(&pool->cq_cached_prod_lock);
-
-	return ret;
-}
-
 static void xsk_cq_submit_addr_locked(struct xsk_buff_pool *pool,
 				      struct sk_buff *skb)
 {
@@ -585,11 +574,9 @@ static void xsk_cq_submit_addr_locked(struct xsk_buff_pool *pool,
 	spin_unlock_irqrestore(&pool->cq_prod_lock, flags);
 }
 
-static void xsk_cq_cancel_locked(struct xsk_buff_pool *pool, u32 n)
+static void xsk_cq_cached_prod_cancel(struct xsk_buff_pool *pool, u32 n)
 {
-	spin_lock(&pool->cq_cached_prod_lock);
 	atomic_sub(n, &pool->cq->cached_prod_atomic);
-	spin_unlock(&pool->cq_cached_prod_lock);
 }
 
 static void xsk_inc_num_desc(struct sk_buff *skb)
@@ -643,7 +630,7 @@ static void xsk_consume_skb(struct sk_buff *skb)
 	}
 
 	skb->destructor = sock_wfree;
-	xsk_cq_cancel_locked(xs->pool, num_descs);
+	xsk_cq_cached_prod_cancel(xs->pool, num_descs);
 	/* Free skb without triggering the perf drop trace */
 	consume_skb(skb);
 	xs->skb = NULL;
@@ -860,7 +847,7 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
 		xskq_cons_release(xs->tx);
 	} else {
 		/* Let application retry */
-		xsk_cq_cancel_locked(xs->pool, 1);
+		xsk_cq_cached_prod_cancel(xs->pool, 1);
 	}
 
 	return ERR_PTR(err);
@@ -898,7 +885,7 @@ static int __xsk_generic_xmit(struct sock *sk)
 		 * if there is space in it. This avoids having to implement
 		 * any buffering in the Tx path.
 		 */
-		err = xsk_cq_reserve_locked(xs->pool);
+		err = xsk_cq_cached_prod_reserve(xs->pool->cq);
 		if (err) {
 			err = -EAGAIN;
 			goto out;
diff --git a/net/xdp/xsk_buff_pool.c b/net/xdp/xsk_buff_pool.c
index 51526034c42a..9539f121b290 100644
--- a/net/xdp/xsk_buff_pool.c
+++ b/net/xdp/xsk_buff_pool.c
@@ -91,7 +91,6 @@ struct xsk_buff_pool *xp_create_and_assign_umem(struct xdp_sock *xs,
 	INIT_LIST_HEAD(&pool->xsk_tx_list);
 	spin_lock_init(&pool->xsk_tx_list_lock);
 	spin_lock_init(&pool->cq_prod_lock);
-	spin_lock_init(&pool->cq_cached_prod_lock);
 	refcount_set(&pool->users, 1);
 
 	pool->fq = xs->fq_tmp;
-- 
2.41.3