1. 08 Nov, 2016 3 commits
    • Jesper Dangaard Brouer's avatar
      qdisc: catch misconfig of attaching qdisc to tx_queue_len zero device · 84c46dd8
      Jesper Dangaard Brouer authored
      It is a clear misconfiguration to attach a qdisc to a device with
      tx_queue_len zero, because some qdisc's (namely, pfifo, bfifo, gred,
      htb, plug and sfb) inherit/copy this value as their queue length.
      
      Why should the kernel catch such a misconfiguration?  Because prior to
      introducing the IFF_NO_QUEUE device flag, userspace found a loophole
      in the qdisc config system that allowed them to achieve the equivalent
      of IFF_NO_QUEUE, which is to remove the qdisc code path entirely from
      a device.  The loophole on older kernels is setting tx_queue_len=0,
      *prior* to device qdisc init (the config time is significant, simply
      setting tx_queue_len=0 doesn't trigger the loophole).
      
      This loophole is currently used by Docker[1] to get better performance
      and scalability out of the veth device.  The Docker developers were
      warned[1] that they needed to adjust the tx_queue_len if ever
      attaching a qdisc.  The OpenShift project didn't remember this warning
      and attached a qdisc, this were caught and fixed in[2].
      
      [1] https://github.com/docker/libcontainer/pull/193
      [2] https://github.com/openshift/origin/pull/11126
      
      Instead of fixing every userspace program that used this loophole, and
      forgot to reset the tx_queue_len, prior to attaching a qdisc.  Let's
      catch the misconfiguration on the kernel side.
      Signed-off-by: default avatarJesper Dangaard Brouer <brouer@redhat.com>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      84c46dd8
    • Jesper Dangaard Brouer's avatar
      net/qdisc: IFF_NO_QUEUE drivers should use consistent TX queue len · 11597084
      Jesper Dangaard Brouer authored
      The flag IFF_NO_QUEUE marks virtual device drivers that doesn't need a
      default qdisc attached, given they will be backed by physical device,
      that already have a qdisc attached for pushback.
      
      It is still supported to attach a qdisc to a IFF_NO_QUEUE device, as
      this can be useful for difference policy reasons (e.g. bandwidth
      limiting containers).  For this to work, the tx_queue_len need to have
      a sane value, because some qdiscs inherit/copy the tx_queue_len
      (namely, pfifo, bfifo, gred, htb, plug and sfb).
      
      Commit a813104d ("IFF_NO_QUEUE: Fix for drivers not calling
      ether_setup()") caught situations where some drivers didn't initialize
      tx_queue_len.  The problem with the commit was choosing 1 as the
      fallback value.
      
      A qdisc queue length of 1 causes more harm than good, because it
      creates hard to debug situations for userspace. It gives userspace a
      false sense of a working config after attaching a qdisc.  As low
      volume traffic (that doesn't activate the qdisc policy) works,
      like ping, while traffic that e.g. needs shaping cannot reach the
      configured policy levels, given the queue length is too small.
      
      This patch change the value to DEFAULT_TX_QUEUE_LEN, given other
      IFF_NO_QUEUE devices (that call ether_setup()) also use this value.
      
      Fixes: a813104d ("IFF_NO_QUEUE: Fix for drivers not calling ether_setup()")
      Signed-off-by: default avatarJesper Dangaard Brouer <brouer@redhat.com>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      11597084
    • Jesper Dangaard Brouer's avatar
      net: make default TX queue length a defined constant · d0a81f67
      Jesper Dangaard Brouer authored
      The default TX queue length of Ethernet devices have been a magic
      constant of 1000, ever since the initial git import.
      
      Looking back in historical trees[1][2] the value used to be 100,
      with the same comment "Ethernet wants good queues". The commit[3]
      that changed this from 100 to 1000 didn't describe why, but from
      conversations with Robert Olsson it seems that it was changed
      when Ethernet devices went from 100Mbit/s to 1Gbit/s, because the
      link speed increased x10 the queue size were also adjusted.  This
      value later caused much heartache for the bufferbloat community.
      
      This patch merely moves the value into a defined constant.
      
      [1] https://git.kernel.org/cgit/linux/kernel/git/davem/netdev-vger-cvs.git/
      [2] https://git.kernel.org/cgit/linux/kernel/git/tglx/history.git/
      [3] https://git.kernel.org/tglx/history/c/98921832c232Signed-off-by: default avatarJesper Dangaard Brouer <brouer@redhat.com>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      d0a81f67
  2. 07 Nov, 2016 21 commits
  3. 05 Nov, 2016 16 commits