Commit a297569f authored Dec 05, 2016 by Eric Dumazet Committed by David S. Miller Dec 06, 2016

net/udp: do not touch skb->peeked unless really needed



In UDP recvmsg() path we currently access 3 cache lines from an skb
while holding receive queue lock, plus another one if packet is
dequeued, since we need to change skb->next->prev

1st cache line (contains ->next/prev pointers, offsets 0x00 and 0x08)
2nd cache line (skb->len & skb->peeked, offsets 0x80 and 0x8e)
3rd cache line (skb->truesize/users, offsets 0xe0 and 0xe4)

skb->peeked is only needed to make sure 0-length packets are properly
handled while MSG_PEEK is operated.

I had first the intent to remove skb->peeked but the "MSG_PEEK at
non-zero offset" support added by Sam Kumar makes this not possible.

This patch avoids one cache line miss during the locked section, when
skb->len and skb->peeked do not have to be read.

It also avoids the skb_set_peeked() cost for non empty UDP datagrams.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent e466af66

net/core/datagram.c

+10 −9

Original line number	Diff line number	Diff line
		@@ -214,6 +214,7 @@ struct sk_buff __skb_try_recv_datagram(struct sock sk, unsigned int flags,
		if (error)
		goto no_packet;

		*peeked = 0;
		do {
		/* Again only user level code calls this function, so nothing
		* interrupt level will suddenly eat the receive_queue.
		@@ -227,22 +228,22 @@ struct sk_buff __skb_try_recv_datagram(struct sock sk, unsigned int flags,
		spin_lock_irqsave(&queue->lock, cpu_flags);
		skb_queue_walk(queue, skb) {
		*last = skb;
		*peeked = skb->peeked;
		if (flags & MSG_PEEK) {
		if (_off >= skb->len && (skb->len \|\| _off \|\|
		skb->peeked)) {
		_off -= skb->len;
		continue;
		}

		if (!skb->len) {
		skb = skb_set_peeked(skb);
		error = PTR_ERR(skb);
		if (IS_ERR(skb)) {
		error = PTR_ERR(skb);
		spin_unlock_irqrestore(&queue->lock,
		cpu_flags);
		goto no_packet;
		}

		}
		*peeked = 1;
		atomic_inc(&skb->users);
		} else {
		__skb_unlink(skb, queue);