net: use llist for sd->defer_list

Get rid of sd->defer_lock and adopt llist operations.

We optimize skb_attempt_defer_free() for the common case,
where the packet is queued. Otherwise sd->defer_count
is increasing, until skb_defer_free_flush() clears it.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250928084934.3266948-3-edumazet@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: edumazet

include/linux/netdevice.h

@@ -3537,10 +3537,10 @@ struct softnet_data {
 	struct numa_drop_counters drop_counters;
 
 	/* Another possibly contended cache line */
-	spinlock_t		defer_lock ____cacheline_aligned_in_smp;
-	atomic_t		defer_count;
-	int			defer_ipi_scheduled;
-	struct sk_buff		*defer_list;
+	struct llist_head	defer_list ____cacheline_aligned_in_smp;
+	atomic_long_t		defer_count;
+
+	int			defer_ipi_scheduled ____cacheline_aligned_in_smp;
 	call_single_data_t	defer_csd;
 };
 

net/core/dev.c

@@ -6717,22 +6717,16 @@ EXPORT_SYMBOL(napi_complete_done);
 
 static void skb_defer_free_flush(struct softnet_data *sd)
 {
+	struct llist_node *free_list;
 	struct sk_buff *skb, *next;
 
-	/* Paired with WRITE_ONCE() in skb_attempt_defer_free() */
-	if (!READ_ONCE(sd->defer_list))
+	if (llist_empty(&sd->defer_list))
 		return;
+	atomic_long_set(&sd->defer_count, 0);
+	free_list = llist_del_all(&sd->defer_list);
 
-	spin_lock(&sd->defer_lock);
-	skb = sd->defer_list;
-	sd->defer_list = NULL;
-	atomic_set(&sd->defer_count, 0);
-	spin_unlock(&sd->defer_lock);
-
-	while (skb != NULL) {
-		next = skb->next;
+	llist_for_each_entry_safe(skb, next, free_list, ll_node) {
 		napi_consume_skb(skb, 1);
-		skb = next;
 	}
 }
 
@@ -12995,7 +12989,7 @@ static int __init net_dev_init(void)
 		sd->cpu = i;
 #endif
 		INIT_CSD(&sd->defer_csd, trigger_rx_softirq, sd);
-		spin_lock_init(&sd->defer_lock);
+		init_llist_head(&sd->defer_list);
 
 		gro_init(&sd->backlog.gro);
 		sd->backlog.poll = process_backlog;

net/core/skbuff.c

@@ -7185,6 +7185,7 @@ static void kfree_skb_napi_cache(struct sk_buff *skb)
  */
 void skb_attempt_defer_free(struct sk_buff *skb)
 {
+	unsigned long defer_count;
 	int cpu = skb->alloc_cpu;
 	struct softnet_data *sd;
 	unsigned int defer_max;
@@ -7202,17 +7203,15 @@ nodefer:	kfree_skb_napi_cache(skb);
 
 	sd = &per_cpu(softnet_data, cpu);
 	defer_max = READ_ONCE(net_hotdata.sysctl_skb_defer_max);
-	if (atomic_read(&sd->defer_count) >= defer_max)
+	defer_count = atomic_long_inc_return(&sd->defer_count);
+
+	if (defer_count >= defer_max)
 		goto nodefer;
 
-	spin_lock_bh(&sd->defer_lock);
-	/* Send an IPI every time queue reaches half capacity. */
-	kick = (atomic_inc_return(&sd->defer_count) - 1) == (defer_max >> 1);
+	llist_add(&skb->ll_node, &sd->defer_list);
 
-	skb->next = sd->defer_list;
-	/* Paired with READ_ONCE() in skb_defer_free_flush() */
-	WRITE_ONCE(sd->defer_list, skb);
-	spin_unlock_bh(&sd->defer_lock);
+	/* Send an IPI every time queue reaches half capacity. */
+	kick = (defer_count - 1) == (defer_max >> 1);
 
 	/* Make sure to trigger NET_RX_SOFTIRQ on the remote CPU
 	 * if we are unlucky enough (this seems very unlikely).