• Eric Dumazet's avatar
    ipv4: PKTINFO doesnt need dst reference · d826eb14
    Eric Dumazet authored
    Le lundi 07 novembre 2011 à 15:33 +0100, Eric Dumazet a écrit :
    
    > At least, in recent kernels we dont change dst->refcnt in forwarding
    > patch (usinf NOREF skb->dst)
    >
    > One particular point is the atomic_inc(dst->refcnt) we have to perform
    > when queuing an UDP packet if socket asked PKTINFO stuff (for example a
    > typical DNS server has to setup this option)
    >
    > I have one patch somewhere that stores the information in skb->cb[] and
    > avoid the atomic_{inc|dec}(dst->refcnt).
    >
    
    OK I found it, I did some extra tests and believe its ready.
    
    [PATCH net-next] ipv4: IP_PKTINFO doesnt need dst reference
    
    When a socket uses IP_PKTINFO notifications, we currently force a dst
    reference for each received skb. Reader has to access dst to get needed
    information (rt_iif & rt_spec_dst) and must release dst reference.
    
    We also forced a dst reference if skb was put in socket backlog, even
    without IP_PKTINFO handling. This happens under stress/load.
    
    We can instead store the needed information in skb->cb[], so that only
    softirq handler really access dst, improving cache hit ratios.
    
    This removes two atomic operations per packet, and false sharing as
    well.
    
    On a benchmark using a mono threaded receiver (doing only recvmsg()
    calls), I can reach 720.000 pps instead of 570.000 pps.
    
    IP_PKTINFO is typically used by DNS servers, and any multihomed aware
    UDP application.
    Signed-off-by: default avatarEric Dumazet <eric.dumazet@gmail.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    d826eb14
ip.h 13.9 KB