path: root/kernel/bpf/core.c
diff options
authorEric Dumazet <>2014-09-08 08:06:07 -0700
committerDavid S. Miller <>2014-09-09 16:54:41 -0700
commitca777eff51f7fbaebd954e645d8ecb781a906b4a (patch)
treeab24468c8118a7ecbef152d7c0c993023f425c1f /kernel/bpf/core.c
parent32bc6d1a35f8897fbcdc260addc1b1ad63b8db15 (diff)
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 <>
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