• Jon Paul Maloy's avatar
    tipc: clean up handling of message priorities · e3eea1eb
    Jon Paul Maloy authored
    Messages transferred by TIPC are assigned an "importance priority", -an
    integer value indicating how to treat the message when there is link or
    destination socket congestion.
    
    There is no separate header field for this value. Instead, the message
    user values have been chosen in ascending order according to perceived
    importance, so that the message user field can be used for this.
    
    This is not a good solution. First, we have many more users than the
    needed priority levels, so we end up with treating more priority
    levels than necessary. Second, the user field cannot always
    accurately reflect the priority of the message. E.g., a message
    fragment packet should really have the priority of the enveloped
    user data message, and not the priority of the MSG_FRAGMENTER user.
    Until now, we have been working around this problem in different ways,
    but it is now time to implement a consistent way of handling such
    priorities, although still within the constraint that we cannot
    allocate any more bits in the regular data message header for this.
    
    In this commit, we define a new priority level, TIPC_SYSTEM_IMPORTANCE,
    that will be the only one used apart from the four (lower) user data
    levels. All non-data messages map down to this priority. Furthermore,
    we take some free bits from the MSG_FRAGMENTER header and allocate
    them to store the priority of the enveloped message. We then adjust
    the functions msg_importance()/msg_set_importance() so that they
    read/set the correct header fields depending on user type.
    
    This small protocol change is fully compatible, because the code at
    the receiving end of a link currently reads the importance level
    only from user data messages, where there is no change.
    Reviewed-by: default avatarErik Hugne <erik.hugne@ericsson.com>
    Signed-off-by: default avatarJon Maloy <jon.maloy@ericsson.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    e3eea1eb
link.c 62 KB