• Jon Paul Maloy's avatar
    tipc: reduce risk of user starvation during link congestion · 365ad353
    Jon Paul Maloy authored
    The socket code currently handles link congestion by either blocking
    and trying to send again when the congestion has abated, or just
    returning to the user with -EAGAIN and let him re-try later.
    
    This mechanism is prone to starvation, because the wakeup algorithm is
    non-atomic. During the time the link issues a wakeup signal, until the
    socket wakes up and re-attempts sending, other senders may have come
    in between and occupied the free buffer space in the link. This in turn
    may lead to a socket having to make many send attempts before it is
    successful. In extremely loaded systems we have observed latency times
    of several seconds before a low-priority socket is able to send out a
    message.
    
    In this commit, we simplify this mechanism and reduce the risk of the
    described scenario happening. When a message is attempted sent via a
    congested link, we now let it be added to the link's backlog queue
    anyway, thus permitting an oversubscription of one message per source
    socket. We still create a wakeup item and return an error code, hence
    instructing the sender to block or stop sending. Only when enough space
    has been freed up in the link's backlog queue do we issue a wakeup event
    that allows the sender to continue with the next message, if any.
    
    The fact that a socket now can consider a message sent even when the
    link returns a congestion code means that the sending socket code can
    be simplified. Also, since this is a good opportunity to get rid of the
    obsolete 'mtu change' condition in the three socket send functions, we
    now choose to refactor those functions completely.
    Signed-off-by: default avatarParthasarathy Bhuvaragan <parthasarathy.bhuvaragan@ericsson.com>
    Acked-by: default avatarYing Xue <ying.xue@windriver.com>
    Signed-off-by: default avatarJon Maloy <jon.maloy@ericsson.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    365ad353
msg.h 20.3 KB