On Tue, 4 Sep 2007 06:35:25 +0100
Stephen Hemminger <[EMAIL PROTECTED]> wrote:
> Andi mentioned he did something like this already, but never
> submitted it.
>
> The dhcp client application uses AF_PACKET with a packet filter to
> receive data. The application doesn't even use timestamps, but because
> the AF_PACKET API has timestamps, they get turned on globally which
> causes an expensive time of day lookup for every packet received
> on any system that uses the standard DHCP client.
>
> The fix is to not enable the timestamp (but use if if available).
> This causes the time lookup to only occur on those packets
> that are destined for the AF_PACKET socket.
> The timestamping occurs after packet filtering
> so all packets dropped by filtering to not cause a clock call.
>
> The one downside of this a a few microseconds additional delay
> added from the normal timestamping location (netif_rx) until the
> receive callback in AF_PACKET. But since the offset is fairly consistent
> it should not upset applications that do want really use timestamps,
> like wireshark.
This patch seems the correct fix for this longstanding problem.
Please note that if wireshark/tcpdump processes really want precise timestamps,
they still can ask this by using setsockopt(SO_TIMESTAMP or SO_TIMESTAMPNS) on
their private socket.
(We already know that running a sniffer has a cost anyway)
>
> Signed-off-by: Stephen Hemminger <[EMAIL PROTECTED]>
>
>
> --- a/net/packet/af_packet.c 2007-07-23 09:31:26.000000000 +0100
> +++ b/net/packet/af_packet.c 2007-09-03 14:55:00.000000000 +0100
> @@ -640,11 +640,10 @@ static int tpacket_rcv(struct sk_buff *s
> h->tp_snaplen = snaplen;
> h->tp_mac = macoff;
> h->tp_net = netoff;
> - if (skb->tstamp.tv64 == 0) {
> - __net_timestamp(skb);
> - sock_enable_timestamp(sk);
> - }
> - tv = ktime_to_timeval(skb->tstamp);
> + if (skb->tstamp.tv64)
> + tv = ktime_to_timeval(skb->tstamp);
> + else
> + do_gettimeofday(&tv);
> h->tp_sec = tv.tv_sec;
> h->tp_usec = tv.tv_usec;
>
-
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