From ca777eff51f7fbaebd954e645d8ecb781a906b4a Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Mon, 8 Sep 2014 08:06:07 -0700 Subject: [PATCH] tcp: remove dst refcount false sharing for prequeue mode Alexander Duyck reported high false sharing on dst refcount in tcp stack when prequeue is used. prequeue is the mechanism used when a thread is blocked in recvmsg()/read() on a TCP socket, using a blocking model rather than select()/poll()/epoll() non blocking one. We already try to use RCU in input path as much as possible, but we were forced to take a refcount on the dst when skb escaped RCU protected region. When/if the user thread runs on different cpu, dst_release() will then touch dst refcount again. Commit 093162553c33 (tcp: force a dst refcount when prequeue packet) was an example of a race fix. It turns out the only remaining usage of skb->dst for a packet stored in a TCP socket prequeue is IP early demux. We can add a logic to detect when IP early demux is probably going to use skb->dst. Because we do an optimistic check rather than duplicate existing logic, we need to guard inet_sk_rx_dst_set() and inet6_sk_rx_dst_set() from using a NULL dst. Many thanks to Alexander for providing a nice bug report, git bisection, and reproducer. Tested using Alexander script on a 40Gb NIC, 8 RX queues. Hosts have 24 cores, 48 hyper threads. echo 0 >/proc/sys/net/ipv4/tcp_autocorking for i in `seq 0 47` do for j in `seq 0 2` do netperf -H $DEST -t TCP_STREAM -l 1000 \ -c -C -T $i,$i -P 0 -- \ -m 64 -s 64K -D & done done Before patch : ~6Mpps and ~95% cpu usage on receiver After patch : ~9Mpps and ~35% cpu usage on receiver. Signed-off-by: Eric Dumazet Reported-by: Alexander Duyck Signed-off-by: David S. Miller --- net/ipv4/tcp_ipv4.c | 20 ++++++++++++++++---- net/ipv6/tcp_ipv6.c | 15 +++++++++------ 2 files changed, 25 insertions(+), 10 deletions(-) diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c index 3f9bc3f0bba0..7881b96d2b72 100644 --- a/net/ipv4/tcp_ipv4.c +++ b/net/ipv4/tcp_ipv4.c @@ -1559,7 +1559,17 @@ bool tcp_prequeue(struct sock *sk, struct sk_buff *skb) skb_queue_len(&tp->ucopy.prequeue) == 0) return false; - skb_dst_force(skb); + /* Before escaping RCU protected region, we need to take care of skb + * dst. Prequeue is only enabled for established sockets. + * For such sockets, we might need the skb dst only to set sk->sk_rx_dst + * Instead of doing full sk_rx_dst validity here, let's perform + * an optimistic check. + */ + if (likely(sk->sk_rx_dst)) + skb_dst_drop(skb); + else + skb_dst_force(skb); + __skb_queue_tail(&tp->ucopy.prequeue, skb); tp->ucopy.memory += skb->truesize; if (tp->ucopy.memory > sk->sk_rcvbuf) { @@ -1765,9 +1775,11 @@ void inet_sk_rx_dst_set(struct sock *sk, const struct sk_buff *skb) { struct dst_entry *dst = skb_dst(skb); - dst_hold(dst); - sk->sk_rx_dst = dst; - inet_sk(sk)->rx_dst_ifindex = skb->skb_iif; + if (dst) { + dst_hold(dst); + sk->sk_rx_dst = dst; + inet_sk(sk)->rx_dst_ifindex = skb->skb_iif; + } } EXPORT_SYMBOL(inet_sk_rx_dst_set); diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c index 5b3c70ff7a72..1835480336ac 100644 --- a/net/ipv6/tcp_ipv6.c +++ b/net/ipv6/tcp_ipv6.c @@ -93,13 +93,16 @@ static struct tcp_md5sig_key *tcp_v6_md5_do_lookup(struct sock *sk, static void inet6_sk_rx_dst_set(struct sock *sk, const struct sk_buff *skb) { struct dst_entry *dst = skb_dst(skb); - const struct rt6_info *rt = (const struct rt6_info *)dst; - dst_hold(dst); - sk->sk_rx_dst = dst; - inet_sk(sk)->rx_dst_ifindex = skb->skb_iif; - if (rt->rt6i_node) - inet6_sk(sk)->rx_dst_cookie = rt->rt6i_node->fn_sernum; + if (dst) { + const struct rt6_info *rt = (const struct rt6_info *)dst; + + dst_hold(dst); + sk->sk_rx_dst = dst; + inet_sk(sk)->rx_dst_ifindex = skb->skb_iif; + if (rt->rt6i_node) + inet6_sk(sk)->rx_dst_cookie = rt->rt6i_node->fn_sernum; + } } static void tcp_v6_hash(struct sock *sk) -- 2.30.2