• Eric Dumazet's avatar
    tcp: switch to GSO being always on · 0a6b2a1d
    Eric Dumazet authored
    Oleksandr Natalenko reported performance issues with BBR without FQ
    packet scheduler that were root caused to lack of SG and GSO/TSO on
    his configuration.
    
    In this mode, TCP internal pacing has to setup a high resolution timer
    for each MSS sent.
    
    We could implement in TCP a strategy similar to the one adopted
    in commit fefa569a
    
     ("net_sched: sch_fq: account for schedule/timers drifts")
    or decide to finally switch TCP stack to a GSO only mode.
    
    This has many benefits :
    
    1) Most TCP developments are done with TSO in mind.
    2) Less high-resolution timers needs to be armed for TCP-pacing
    3) GSO can benefit of xmit_more hint
    4) Receiver GRO is more effective (as if TSO was used for real on sender)
       -> Lower ACK traffic
    5) Write queues have less overhead (one skb holds about 64KB of payload)
    6) SACK coalescing just works.
    7) rtx rb-tree contains less packets, SACK is cheaper.
    
    This patch implements the minimum patch, but we can remove some legacy
    code as follow ups.
    
    Tested:
    
    On 40Gbit link, one netperf -t TCP_STREAM
    
    BBR+fq:
    sg on:  26 Gbits/sec
    sg off: 15.7 Gbits/sec   (was 2.3 Gbit before patch)
    
    BBR+pfifo_fast:
    sg on:  24.2 Gbits/sec
    sg off: 14.9 Gbits/sec  (was 0.66 Gbit before patch !!! )
    
    BBR+fq_codel:
    sg on:  24.4 Gbits/sec
    sg off: 15 Gbits/sec  (was 0.66 Gbit before patch !!! )
    Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
    Reported-by: default avatarOleksandr Natalenko <oleksandr@natalenko.name>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    0a6b2a1d