1. 15 Jun, 2016 1 commit
    • Jon Paul Maloy's avatar
      tipc: add neighbor monitoring framework · 35c55c98
      Jon Paul Maloy authored
      
      
      TIPC based clusters are by default set up with full-mesh link
      connectivity between all nodes. Those links are expected to provide
      a short failure detection time, by default set to 1500 ms. Because
      of this, the background load for neighbor monitoring in an N-node
      cluster increases with a factor N on each node, while the overall
      monitoring traffic through the network infrastructure increases at
      a ~(N * (N - 1)) rate. Experience has shown that such clusters don't
      scale well beyond ~100 nodes unless we significantly increase failure
      discovery tolerance.
      
      This commit introduces a framework and an algorithm that drastically
      reduces this background load, while basically maintaining the original
      failure detection times across the whole cluster. Using this algorithm,
      background load will now grow at a rate of ~(2 * sqrt(N)) per node, and
      at ~(2 * N * sqrt(N)) in traffic overhead. As an example, each node will
      now have to actively monitor 38 neighbors in a 400-node cluster, instead
      of as before 399.
      
      This "Overlapping Ring Supervision Algorithm" is completely distributed
      and employs no centralized or coordinated state. It goes as follows:
      
      - Each node makes up a linearly ascending, circular list of all its N
        known neighbors, based on their TIPC node identity. This algorithm
        must be the same on all nodes.
      
      - The node then selects the next M = sqrt(N) - 1 nodes downstream from
        itself in the list, and chooses to actively monitor those. This is
        called its "local monitoring domain".
      
      - It creates a domain record describing the monitoring domain, and
        piggy-backs this in the data area of all neighbor monitoring messages
        (LINK_PROTOCOL/STATE) leaving that node. This means that all nodes in
        the cluster eventually (default within 400 ms) will learn about
        its monitoring domain.
      
      - Whenever a node discovers a change in its local domain, e.g., a node
        has been added or has gone down, it creates and sends out a new
        version of its node record to inform all neighbors about the change.
      
      - A node receiving a domain record from anybody outside its local domain
        matches this against its own list (which may not look the same), and
        chooses to not actively monitor those members of the received domain
        record that are also present in its own list. Instead, it relies on
        indications from the direct monitoring nodes if an indirectly
        monitored node has gone up or down. If a node is indicated lost, the
        receiving node temporarily activates its own direct monitoring towards
        that node in order to confirm, or not, that it is actually gone.
      
      - Since each node is actively monitoring sqrt(N) downstream neighbors,
        each node is also actively monitored by the same number of upstream
        neighbors. This means that all non-direct monitoring nodes normally
        will receive sqrt(N) indications that a node is gone.
      
      - A major drawback with ring monitoring is how it handles failures that
        cause massive network partitionings. If both a lost node and all its
        direct monitoring neighbors are inside the lost partition, the nodes in
        the remaining partition will never receive indications about the loss.
        To overcome this, each node also chooses to actively monitor some
        nodes outside its local domain. Those nodes are called remote domain
        "heads", and are selected in such a way that no node in the cluster
        will be more than two direct monitoring hops away. Because of this,
        each node, apart from monitoring the member of its local domain, will
        also typically monitor sqrt(N) remote head nodes.
      
      - As an optimization, local list status, domain status and domain
        records are marked with a generation number. This saves senders from
        unnecessarily conveying  unaltered domain records, and receivers from
        performing unneeded re-adaptations of their node monitoring list, such
        as re-assigning domain heads.
      
      - As a measure of caution we have added the possibility to disable the
        new algorithm through configuration. We do this by keeping a threshold
        value for the cluster size; a cluster that grows beyond this value
        will switch from full-mesh to ring monitoring, and vice versa when
        it shrinks below the value. This means that if the threshold is set to
        a value larger than any anticipated cluster size (default size is 32)
        the new algorithm is effectively disabled. A patch set for altering the
        threshold value and for listing the table contents will follow shortly.
      
      - This change is fully backwards compatible.
      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>
      35c55c98
  2. 11 Apr, 2016 1 commit
  3. 20 Nov, 2015 1 commit
  4. 24 Oct, 2015 4 commits
    • Jon Paul Maloy's avatar
      tipc: clean up unused code and structures · 2af5ae37
      Jon Paul Maloy authored
      
      
      After the previous changes in this series, we can now remove some
      unused code and structures, both in the broadcast, link aggregation
      and link code.
      
      There are no functional changes in this commit.
      Signed-off-by: default avatarJon Maloy <jon.maloy@ericsson.com>
      Reviewed-by: default avatarYing Xue <ying.xue@windriver.com>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      2af5ae37
    • Jon Paul Maloy's avatar
      tipc: let broadcast packet reception use new link receive function · 52666986
      Jon Paul Maloy authored
      
      
      The code path for receiving broadcast packets is currently distinct
      from the unicast path. This leads to unnecessary code and data
      duplication, something that can be avoided with some effort.
      
      We now introduce separate per-peer tipc_link instances for handling
      broadcast packet reception. Each receive link keeps a pointer to the
      common, single, broadcast link instance, and can hence handle release
      and retransmission of send buffers as if they belonged to the own
      instance.
      
      Furthermore, we let each unicast link instance keep a reference to both
      the pertaining broadcast receive link, and to the common send link.
      This makes it possible for the unicast links to easily access data for
      broadcast link synchronization, as well as for carrying acknowledges for
      received broadcast packets.
      Signed-off-by: default avatarJon Maloy <jon.maloy@ericsson.com>
      Reviewed-by: default avatarYing Xue <ying.xue@windriver.com>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      52666986
    • Jon Paul Maloy's avatar
      tipc: move broadcast link lock to struct tipc_net · 0043550b
      Jon Paul Maloy authored
      
      
      The broadcast lock will need to be acquired outside bcast.c in a later
      commit. For this reason, we move the lock to struct tipc_net. Consistent
      with the changes in the previous commit, we also introducee two new
      functions tipc_bcast_lock() and tipc_bcast_unlock(). The code that is
      currently using tipc_bclink_lock()/unlock() will be phased out during
      the coming commits in this series.
      Signed-off-by: default avatarJon Maloy <jon.maloy@ericsson.com>
      Reviewed-by: default avatarYing Xue <ying.xue@windriver.com>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      0043550b
    • Jon Paul Maloy's avatar
      tipc: move bcast definitions to bcast.c · 6beb19a6
      Jon Paul Maloy authored
      
      
      Currently, a number of structure and function definitions related
      to the broadcast functionality are unnecessarily exposed in the file
      bcast.h. This obscures the fact that the external interface towards
      the broadcast link in fact is very narrow, and causes unnecessary
      recompilations of other files when anything changes in those
      definitions.
      
      In this commit, we move as many of those definitions as is currently
      possible to the file bcast.c.
      
      We also rename the structure 'tipc_bclink' to 'tipc_bc_base', both
      since the name does not correctly describe the contents of this
      struct, and will do so even less in the future, and because we want
      to use the term 'link' more appropriately in the functionality
      introduced later in this series.
      
      Finally, we rename a couple of functions, such as tipc_bclink_xmit()
      and others that will be kept in the future, to include the term 'bcast'
      instead.
      
      There are no functional changes in this commit.
      Signed-off-by: default avatarJon Maloy <jon.maloy@ericsson.com>
      Reviewed-by: default avatarYing Xue <ying.xue@windriver.com>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      6beb19a6
  5. 31 Jul, 2015 1 commit
  6. 21 Jul, 2015 1 commit
    • Jon Paul Maloy's avatar
      tipc: reduce locking scope during packet reception · d999297c
      Jon Paul Maloy authored
      
      
      We convert packet/message reception according to the same principle
      we have been using for message sending and timeout handling:
      
      We move the function tipc_rcv() to node.c, hence handling the initial
      packet reception at the link aggregation level. The function grabs
      the node lock, selects the receiving link, and accesses it via a new
      call tipc_link_rcv(). This function appends buffers to the input
      queue for delivery upwards, but it may also append outgoing packets
      to the xmit queue, just as we do during regular message sending. The
      latter will happen when buffers are forwarded from the link backlog,
      or when retransmission is requested.
      
      Upon return of this function, and after having released the node lock,
      tipc_rcv() delivers/tranmsits the contents of those queues, but it may
      also perform actions such as link activation or reset, as indicated by
      the return flags from the link.
      
      This reduces the number of cpu cycles spent inside the node spinlock,
      and reduces contention on that lock.
      Reviewed-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>
      d999297c
  7. 14 May, 2015 2 commits
    • Jon Paul Maloy's avatar
      tipc: simplify packet sequence number handling · e4bf4f76
      Jon Paul Maloy authored
      
      
      Although the sequence number in the TIPC protocol is 16 bits, we have
      until now stored it internally as an unsigned 32 bits integer.
      We got around this by always doing explicit modulo-65535 operations
      whenever we need to access a sequence number.
      
      We now make the incoming and outgoing sequence numbers to unsigned
      16-bit integers, and remove the modulo operations where applicable.
      
      We also move the arithmetic inline functions for 16 bit integers
      to core.h, and the function buf_seqno() to msg.h, so they can easily
      be accessed from anywhere in the code.
      Reviewed-by: default avatarErik Hugne <erik.hugne@ericsson.com>
      Reviewed-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>
      e4bf4f76
    • Jon Paul Maloy's avatar
      tipc: simplify include dependencies · a6bf70f7
      Jon Paul Maloy authored
      
      
      When we try to add new inline functions in the code, we sometimes
      run into circular include dependencies.
      
      The main problem is that the file core.h, which really should be at
      the root of the dependency chain, instead is a leaf. I.e., core.h
      includes a number of header files that themselves should be allowed
      to include core.h. In reality this is unnecessary, because core.h does
      not need to know the full signature of any of the structs it refers to,
      only their type declaration.
      
      In this commit, we remove all dependencies from core.h towards any
      other tipc header file.
      
      As a consequence of this change, we can now move the function
      tipc_own_addr(net) from addr.c to addr.h, and make it inline.
      
      There are no functional changes in this commit.
      Reviewed-by: default avatarErik Hugne <erik.hugne@ericsson.com>
      Reviewed-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>
      a6bf70f7
  8. 09 Feb, 2015 2 commits
  9. 12 Jan, 2015 11 commits
  10. 09 Jan, 2015 1 commit
    • Ying Xue's avatar
      tipc: convert tipc reference table to use generic rhashtable · 07f6c4bc
      Ying Xue authored
      
      
      As tipc reference table is statically allocated, its memory size
      requested on stack initialization stage is quite big even if the
      maximum port number is just restricted to 8191 currently, however,
      the number already becomes insufficient in practice. But if the
      maximum ports is allowed to its theory value - 2^32, its consumed
      memory size will reach a ridiculously unacceptable value. Apart from
      this, heavy tipc users spend a considerable amount of time in
      tipc_sk_get() due to the read-lock on ref_table_lock.
      
      If tipc reference table is converted with generic rhashtable, above
      mentioned both disadvantages would be resolved respectively: making
      use of the new resizable hash table can avoid locking on the lookup;
      smaller memory size is required at initial stage, for example, 256
      hash bucket slots are requested at the beginning phase instead of
      allocating the entire 8191 slots in old mode. The hash table will
      grow if entries exceeds 75% of table size up to a total table size
      of 1M, and it will automatically shrink if usage falls below 30%,
      but the minimum table size is allowed down to 256.
      
      Also converts ref_table_lock to a separate mutex to protect hash table
      mutations on write side. Lastly defers the release of the socket
      reference using call_rcu() to allow using an RCU read-side protected
      call to rhashtable_lookup().
      Signed-off-by: default avatarYing Xue <ying.xue@windriver.com>
      Acked-by: default avatarJon Maloy <jon.maloy@ericsson.com>
      Acked-by: default avatarErik Hugne <erik.hugne@ericsson.com>
      Cc: Thomas Graf <tgraf@suug.ch>
      Acked-by: default avatarThomas Graf <tgraf@suug.ch>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      07f6c4bc
  11. 26 Nov, 2014 1 commit
  12. 21 Nov, 2014 1 commit
    • Richard Alpe's avatar
      tipc: add bearer disable/enable to new netlink api · 0655f6a8
      Richard Alpe authored
      
      
      A new netlink API for tipc that can disable or enable a tipc bearer.
      
      The new API is separated from the old API because of a bug in the
      user space client (tipc-config). The problem is that older versions
      of tipc-config has a very low receive limit and adding commands to
      the legacy genl_opts struct causes the ctrl_getfamily() response
      message to grow, subsequently breaking the tool.
      
      The new API utilizes netlink policies for input validation. Where the
      top-level netlink attributes are tipc-logical entities, like bearer.
      The top level entities then contain nested attributes. In this case
      a name, nested link properties and a domain.
      
      Netlink commands implemented in this patch:
      TIPC_NL_BEARER_ENABLE
      TIPC_NL_BEARER_DISABLE
      
      Netlink logical layout of bearer enable message:
      -> bearer
          -> name
          [ -> domain ]
          [
          -> properties
              -> priority
          ]
      
      Netlink logical layout of bearer disable message:
      -> bearer
          -> name
      Signed-off-by: default avatarRichard Alpe <richard.alpe@ericsson.com>
      Reviewed-by: default avatarErik Hugne <erik.hugne@ericsson.com>
      Reviewed-by: default avatarJon Maloy <jon.maloy@ericsson.com>
      Acked-by: default avatarYing Xue <ying.xue@windriver.com>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      0655f6a8
  13. 02 Sep, 2014 1 commit
    • Erik Hugne's avatar
      tipc: add name distributor resiliency queue · a5325ae5
      Erik Hugne authored
      
      
      TIPC name table updates are distributed asynchronously in a cluster,
      entailing a risk of certain race conditions. E.g., if two nodes
      simultaneously issue conflicting (overlapping) publications, this may
      not be detected until both publications have reached a third node, in
      which case one of the publications will be silently dropped on that
      node. Hence, we end up with an inconsistent name table.
      
      In most cases this conflict is just a temporary race, e.g., one
      node is issuing a publication under the assumption that a previous,
      conflicting, publication has already been withdrawn by the other node.
      However, because of the (rtt related) distributed update delay, this
      may not yet hold true on all nodes. The symptom of this failure is a
      syslog message: "tipc: Cannot publish {%u,%u,%u}, overlap error".
      
      In this commit we add a resiliency queue at the receiving end of
      the name table distributor. When insertion of an arriving publication
      fails, we retain it in this queue for a short amount of time, assuming
      that another update will arrive very soon and clear the conflict. If so
      happens, we insert the publication, otherwise we drop it.
      
      The (configurable) retention value defaults to 2000 ms. Knowing from
      experience that the situation described above is extremely rare, there
      is no risk that the queue will accumulate any large number of items.
      Signed-off-by: default avatarErik Hugne <erik.hugne@ericsson.com>
      Signed-off-by: default avatarJon Maloy <jon.maloy@ericsson.com>
      Acked-by: default avatarYing Xue <ying.xue@windriver.com>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      a5325ae5
  14. 23 Aug, 2014 1 commit
    • Jon Paul Maloy's avatar
      tipc: use pseudo message to wake up sockets after link congestion · 50100a5e
      Jon Paul Maloy authored
      
      
      The current link implementation keeps a linked list of blocked ports/
      sockets that is populated when there is link congestion. The purpose
      of this is to let the link know which users to wake up when the
      congestion abates.
      
      This adds unnecessary complexity to the data structure and the code,
      since it forces us to involve the link each time we want to delete
      a socket. It also forces us to grab the spinlock port_lock within
      the scope of node_lock. We want to get rid of this direct dependence,
      as well as the deadlock hazard resulting from the usage of port_lock.
      
      In this commit, we instead let the link keep list of a "wakeup" pseudo
      messages for use in such situations. Those messages are sent to the
      pending sockets via the ordinary message reception path, and wake up
      the socket's owner when they are received.
      
      This enables us to get rid of the 'waiting_ports' linked lists in struct
      tipc_port that manifest this direct reference. As a consequence, we can
      eliminate another BH entry into the socket, and hence the need to grab
      port_lock. This is a further step in our effort to remove port_lock
      altogether.
      Signed-off-by: default avatarJon Maloy <jon.maloy@ericsson.com>
      Reviewed-by: default avatarErik Hugne <erik.hugne@ericsson.com>
      Reviewed-by: default avatarYing Xue <ying.xue@windriver.com>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      50100a5e
  15. 14 May, 2014 2 commits
    • Jon Paul Maloy's avatar
      tipc: improve and extend media address conversion functions · 38504c28
      Jon Paul Maloy authored
      
      
      TIPC currently handles two media specific addresses: Ethernet MAC
      addresses and InfiniBand addresses. Those are kept in three different
      formats:
      
      1) A "raw" format as obtained from the device. This format is known
         only by the media specific adapter code in eth_media.c and
         ib_media.c.
      2) A "generic" internal format, in the form of struct tipc_media_addr,
         which can be referenced and passed around by the generic media-
         unaware code.
      3) A serialized version of the latter, to be conveyed in neighbor
         discovery messages.
      
      Conversion between the three formats can only be done by the media
      specific code, so we have function pointers for this purpose in
      struct tipc_media. Here, the media adapters can install their own
      conversion functions at startup.
      
      We now introduce a new such function, 'raw2addr()', whose purpose
      is to convert from format 1 to format 2 above. We also try to as far
      as possible uniform commenting, variable names and usage of these
      functions, with the purpose of making them more comprehensible.
      
      We can now also remove the function tipc_l2_media_addr_set(), whose
      job is done better by the new function.
      
      Finally, we expand the field for serialized addresses (format 3)
      in discovery messages from 20 to 32 bytes. This is permitted
      according to the spec, and reduces the risk of problems when we
      add new media in the future.
      Signed-off-by: default avatarJon Maloy <jon.maloy@ericsson.com>
      Reviewed-by: default avatarYing Xue <ying.xue@windriver.com>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      38504c28
    • Jon Paul Maloy's avatar
      tipc: rename and move message reassembly function · 37e22164
      Jon Paul Maloy authored
      
      
      The function tipc_link_frag_rcv() is in reality a re-entrant generic
      message reassemby function that has nothing in particular to do with
      the link, where it is defined now. This becomes obvious when we see
      the need to call the function from other places in the code.
      
      In this commit rename it to tipc_buf_append() and move it to the file
      msg.c. We also simplify its signature by moving the tail pointer to
      the control block of the head buffer, hence making the head buffer
      self-contained.
      Signed-off-by: default avatarJon Maloy <jon.maloy@ericsson.com>
      Reviewed-by: default avatarYing Xue <ying.xue@windriver.com>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      37e22164
  16. 05 May, 2014 1 commit
  17. 23 Apr, 2014 1 commit
  18. 27 Mar, 2014 1 commit
  19. 22 Feb, 2014 1 commit
    • Ying Xue's avatar
      tipc: make bearer set up in module insertion stage · 970122fd
      Ying Xue authored
      Accidentally a side effect is involved by commit 6e967adf
      
      (tipc:
      relocate common functions from media to bearer). Now tipc stack
      handler of receiving packets from netdevices as well as netdevice
      notification handler are registered when bearer is enabled rather
      than tipc module initialization stage, but the two handlers are
      both unregistered in tipc module exit phase. If tipc module is
      inserted and then immediately removed, the following warning
      message will appear:
      
      "dev_remove_pack: ffffffffa0380940 not found"
      
      This is because in module insertion stage tipc stack packet handler
      is not registered at all, but in module exit phase dev_remove_pack()
      needs to remove it. Of course, dev_remove_pack() cannot find tipc
      protocol handler from the kernel protocol handler list so that the
      warning message is printed out.
      
      But if registering the two handlers is adjusted from enabling bearer
      phase into inserting module stage, the warning message will be
      eliminated. Due to this change, tipc_core_start_net() and
      tipc_core_stop_net() can be deleted as well.
      Reported-by: default avatarWang Weidong <wangweidong1@huawei.com>
      Cc: Jon Maloy <jon.maloy@ericsson.com>
      Cc: Erik Hugne <erik.hugne@ericsson.com>
      Signed-off-by: default avatarYing Xue <ying.xue@windriver.com>
      Reviewed-by: default avatarPaul Gortmaker <paul.gortmaker@windriver.com>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      970122fd
  20. 13 Feb, 2014 1 commit
  21. 16 Dec, 2013 1 commit
  22. 19 Oct, 2013 1 commit
  23. 17 Jun, 2013 2 commits
    • Ying Xue's avatar
      tipc: introduce new TIPC server infrastructure · c5fa7b3c
      Ying Xue authored
      
      
      TIPC has two internal servers, one providing a subscription
      service for topology events, and another providing the
      configuration interface. These servers have previously been running
      in BH context, accessing the TIPC-port (aka native) API directly.
      Apart from these servers, even the TIPC socket implementation is
      partially built on this API.
      
      As this API may simultaneously be called via different paths and in
      different contexts, a complex and costly lock policiy is required
      in order to protect TIPC internal resources.
      
      To eliminate the need for this complex lock policiy, we introduce
      a new, generic service API that uses kernel sockets for message
      passing instead of the native API. Once the toplogy and configuration
      servers are converted to use this new service, all code pertaining
      to the native API can be removed. This entails a significant
      reduction in code amount and complexity, and opens up for a complete
      rework of the locking policy in TIPC.
      
      The new service also solves another problem:
      
      As the current topology server works in BH context, it cannot easily
      be blocked when sending of events fails due to congestion. In such
      cases events may have to be silently dropped, something that is
      unacceptable. Therefore, the new service keeps a dedicated outbound
      queue receiving messages from BH context. Once messages are
      inserted into this queue, we will immediately schedule a work from a
      special workqueue. This way, messages/events from the topology server
      are in reality sent in process context, and the server can block
      if necessary.
      
      Analogously, there is a new workqueue for receiving messages. Once a
      notification about an arriving message is received in BH context, we
      schedule a work from the receive workqueue to do the job of
      receiving the message in process context.
      
      As both sending and receive messages are now finished in processes,
      subscribed events cannot be dropped any more.
      
      As of this commit, this new server infrastructure is built, but
      not actually yet called by the existing TIPC code, but since the
      conversion changes required in order to use it are significant,
      the addition is kept here as a separate commit.
      Signed-off-by: default avatarYing Xue <ying.xue@windriver.com>
      Signed-off-by: default avatarJon Maloy <jon.maloy@ericsson.com>
      Signed-off-by: default avatarPaul Gortmaker <paul.gortmaker@windriver.com>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      c5fa7b3c
    • Ying Xue's avatar
      tipc: change socket buffer overflow control to respect sk_rcvbuf · cc79dd1b
      Ying Xue authored
      
      
      As per feedback from the netdev community, we change the buffer
      overflow protection algorithm in receiving sockets so that it
      always respects the nominal upper limit set in sk_rcvbuf.
      
      Instead of scaling up from a small sk_rcvbuf value, which leads to
      violation of the configured sk_rcvbuf limit, we now calculate the
      weighted per-message limit by scaling down from a much bigger value,
      still in the same field, according to the importance priority of the
      received message.
      
      To allow for administrative tunability of the socket receive buffer
      size, we create a tipc_rmem sysctl variable to allow the user to
      configure an even bigger value via sysctl command.  It is a size of
      three (min/default/max) to be consistent with things like tcp_rmem.
      
      By default, the value initialized in tipc_rmem[1] is equal to the
      receive socket size needed by a TIPC_CRITICAL_IMPORTANCE message.
      This value is also set as the default value of sk_rcvbuf.
      Originally-by: default avatarJon Maloy <jon.maloy@ericsson.com>
      Cc: Neil Horman <nhorman@tuxdriver.com>
      Cc: Jon Maloy <jon.maloy@ericsson.com>
      [Ying: added sysctl variation to Jon's original patch]
      Signed-off-by: default avatarYing Xue <ying.xue@windriver.com>
      [PG: don't compile sysctl.c if not config'd; add Documentation]
      Signed-off-by: default avatarPaul Gortmaker <paul.gortmaker@windriver.com>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      cc79dd1b