• Eric Dumazet's avatar
    net: sched: fix reordering issues · b88dd52c
    Eric Dumazet authored
    Whenever MQ is not used on a multiqueue device, we experience
    serious reordering problems. Bisection found the cited
    commit.
    
    The issue can be described this way :
    
    - A single qdisc hierarchy is shared by all transmit queues.
      (eg : tc qdisc replace dev eth0 root fq_codel)
    
    - When/if try_bulk_dequeue_skb_slow() dequeues a packet targetting
      a different transmit queue than the one used to build a packet train,
      we stop building the current list and save the 'bad' skb (P1) in a
      special queue. (bad_txq)
    
    - When dequeue_skb() calls qdisc_dequeue_skb_bad_txq() and finds this
      skb (P1), it checks if the associated transmit queues is still in frozen
      state. If the queue is still blocked (by BQL or NIC tx ring full),
      we leave the skb in bad_txq and return NULL.
    
    - dequeue_skb() calls q->dequeue() to get another packet (P2)
    
      The other packet can target the problematic queue (that we found
      in frozen state for the bad_txq packet), but another cpu just ran
      TX completion and made room in the txq that is now ready to accept
      new packets.
    
    - Packet P2 is sent while P1 is still held in bad_txq, P1 might be sent
      at next round. In practice P2 is the lead of a big packet train
      (P2,P3,P4 ...) filling the BQL budget and delaying P1 by many packets :/
    
    To solve this problem, we have to block the dequeue process as long
    as the first packet in bad_txq can not be sent. Reordering issues
    disappear and no side effects have been seen.
    
    Fixes: a53851e2
    
     ("net: sched: explicit locking in gso_cpu fallback")
    Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
    Cc: John Fastabend <john.fastabend@gmail.com>
    Acked-by: default avatarJohn Fastabend <john.fastabend@gmail.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    b88dd52c