udp: Add lockless transmit path (903ab86d) · Commits · 戴 / test

The UDP transmit path has been running under the socket lock for a long time because of the corking feature. This means that transmitting to the same socket in multiple threads does not scale at all. However, as most users don't actually use corking, the locking can be removed in the common case. This patch creates a lockless fast path where corking is not used. Please note that this does create a slight inaccuracy in the enforcement of socket send buffer limits. In particular, we may exceed the socket limit by up to (number of CPUs) * (packet size) because of the way the limit is computed. As the primary purpose of socket buffers is to indicate congestion, this should not be a great problem for now. Signed-off-by:

Herbert Xu <herbert@gondor.apana.org.au> Acked-by:

Eric Dumazet <eric.dumazet@gmail.com> Signed-off-by:

David S. Miller <davem@davemloft.net>

net/ipv4/udp.c

+14 −1

Original line number	Diff line number	Diff line
		@@ -802,6 +802,7 @@ int udp_sendmsg(struct kiocb iocb, struct sock sk, struct msghdr *msg,
		int err, is_udplite = IS_UDPLITE(sk);
		int corkreq = up->corkflag \|\| msg->msg_flags&MSG_MORE;
		int (getfrag)(void , char , int, int, int, struct sk_buff );
		struct sk_buff *skb;

		if (len > 0xFFFF)
		return -EMSGSIZE;
		@@ -816,6 +817,8 @@ int udp_sendmsg(struct kiocb iocb, struct sock sk, struct msghdr *msg,
		ipc.opt = NULL;
		ipc.tx_flags = 0;

		getfrag = is_udplite ? udplite_getfrag : ip_generic_getfrag;

		if (up->pending) {
		/*
		* There are pending frames.
		@@ -940,6 +943,17 @@ back_from_confirm:
		if (!ipc.addr)
		daddr = ipc.addr = rt->rt_dst;

		/* Lockless fast path for the non-corking case. */
		if (!corkreq) {
		skb = ip_make_skb(sk, getfrag, msg->msg_iov, ulen,
		sizeof(struct udphdr), &ipc, &rt,
		msg->msg_flags);
		err = PTR_ERR(skb);
		if (skb && !IS_ERR(skb))
		err = udp_send_skb(skb, daddr, dport);
		goto out;
		}

		lock_sock(sk);
		if (unlikely(up->pending)) {
		/* The socket is already corked while preparing it. */
		@@ -961,7 +975,6 @@ back_from_confirm:

		do_append_data:
		up->len += ulen;
		getfrag = is_udplite ? udplite_getfrag : ip_generic_getfrag;
		err = ip_append_data(sk, getfrag, msg->msg_iov, ulen,
		sizeof(struct udphdr), &ipc, &rt,
		corkreq ? msg->msg_flags\|MSG_MORE : msg->msg_flags);

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