• Eric Dumazet's avatar
    tcp: autocork should not hold first packet in write queue · a181ceb5
    Eric Dumazet authored
    Willem noticed a TCP_RR regression caused by TCP autocorking
    on a Mellanox test bed. MLX4_EN_TX_COAL_TIME is 16 us, which can be
    right above RTT between hosts.
    
    We can receive a ACK for a packet still in NIC TX ring buffer or in a
    softnet completion queue.
    
    Fix this by always pushing the skb if it is at the head of write queue.
    
    Also, as TX completion is lockless, it's safer to perform sk_wmem_alloc
    test after setting TSQ_THROTTLED.
    
    erd:~# MIB="MIN_LATENCY,MEAN_LATENCY,MAX_LATENCY,P99_LATENCY,STDDEV_LATENCY"
    erd:~#  ./netperf -H remote -t TCP_RR -- -o $MIB | tail -n 1
    (repeat 3 times)
    
    Before patch :
    
    18,1049.87,41004,39631,6295.47
    17,239.52,40804,48,2912.79
    18,348.40,40877,54,3573.39
    
    After patch :
    
    18,22.84,4606,38,16.39
    17,21.56,2871,36,13.51
    17,22.46,2705,37,11.83
    Reported-by: default avatarWillem de Bruijn <willemb@google.com>
    Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
    Fixes: f54b3111 ("tcp: auto corking")
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    a181ceb5
tcp.c 82.9 KB