net: prefech skb->priority in __dev_xmit_skb()

mainline inclusion
from mainline-v6.19-rc1
category: performance

Most qdiscs need to read skb->priority at enqueue time().

In commit 100dfa7 ("net: dev_queue_xmit() llist adoption")
I added a prefetch(next), lets add another one for the second
half of skb.

Note that skb->priority and skb->hash share a common cache line,
so this patch helps qdiscs needing both fields.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20251121083256.674562-11-edumazet@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

(cherry picked from commit b2e9821)
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>

Author: edumazet

net/core/dev.c

@@ -4260,6 +4260,7 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
 
 		llist_for_each_entry_safe(skb, next, ll_list, ll_node) {
 			prefetch(next);
+			prefetch(&next->priority);
 			skb_mark_not_on_list(skb);
 			rc = dev_qdisc_enqueue(skb, q, &to_free, txq);
 			count++;