• Jon Maxwell's avatar
    tcp: v1 always send a quick ack when quickacks are enabled · 2251ae46
    Jon Maxwell authored
    V1 of this patch contains Eric Dumazet's suggestion to move the per
    dst RTAX_QUICKACK check into tcp_in_quickack_mode(). Thanks Eric.
    
    I ran some tests and after setting the "ip route change quickack 1"
    knob there were still many delayed ACKs sent. This occured
    because when icsk_ack.quick=0 the !icsk_ack.pingpong value is
    subsequently ignored as tcp_in_quickack_mode() checks both these
    values. The condition for a quick ack to trigger requires
    that both icsk_ack.quick != 0 and icsk_ack.pingpong=0. Currently
    only icsk_ack.pingpong is controlled by the knob. But the
    icsk_ack.quick value changes dynamically depending on heuristics.
    The crux of the matter is that delayed acks still cannot be entirely
    disabled even with the RTAX_QUICKACK per dst knob enabled. This
    patch ensures that a quick ack is always sent when the RTAX_QUICKACK
    per dst knob is turned on.
    
    The "ip route change quickack 1" knob was recently added to enable
    quickacks. It was modeled around the TCP_QUICKACK setsockopt() option.
    This issue is that even with "ip route change quickack 1" enabled
    we still see delayed ACKs under some conditions. It would be nice
    to be able to completely disable delayed ACKs.
    
    Here is an example:
    
    # netstat -s|grep dela
        3 delayed acks sent
    
    For all routes enable the knob
    
    # ip route change quickack 1
    
    Generate some traffic across a slow link and we still see the delayed
    acks.
    
    # netstat -s|grep dela
        106 delayed acks sent
        1 delayed acks further delayed because of locked socket
    
    The issue is that both the "ip route change quickack 1" knob and
    the TCP_QUICKACK option set the icsk_ack.pingpong variable to 0.
    However at the business end in the __tcp_ack_snd_check() routine,
    tcp_in_quickack_mode() checks that both icsk_ack.quick != 0
    and icsk_ack.pingpong=0 in order to trigger a quickack. As
    icsk_ack.quick is determined by heuristics it can be 0. When
    that occurs the icsk_ack.pingpong value is ignored and a delayed
    ACK is sent regardless.
    
    This patch moves the RTAX_QUICKACK per dst check into the
    tcp_in_quickack_mode() routine which ensures that a quickack is
    always sent when the quickack knob is enabled for that dst.
    Signed-off-by: default avatarJon Maxwell <jmaxwell37@gmail.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    2251ae46
tcp_input.c 177 KB