On Tue, Aug 12, 2014 at 06:14:43AM -0700, Eric Dumazet wrote:
> On Tue, 2014-08-12 at 16:33 +0400, Andrew Vagin wrote:
> > On Tue, Aug 12, 2014 at 05:15:01AM -0700, Eric Dumazet wrote:
> > > On Tue, 2014-08-12 at 13:45 +0400, Andrey Vagin wrote:
> > > > We don't know right timestamp for repaired skb-s. Wrong RTT estimations
> > > > isn't good, because some congestion modules heavily depends on it.
> > > >
> > > > This patch adds the TCPCB_REPAIRED flag, which is included in
> > > > TCPCB_RETRANS.
> > >
> > > ...
> > >
> > > > +
> > > > + /* All packets are restored as if they
> > > > have
> > > > + * already been sent. skb_mstamp isn't
> > > > set to
> > > > + * avoid wrong rtt estimation.
> > > > + */
> > > > + if (tp->repair) {
> > > > + TCP_SKB_CB(skb)->sacked |=
> > > > TCPCB_REPAIRED;
> > > > + TCP_SKB_CB(skb)->when =
> > > > tcp_time_stamp;
> > > > + }
> > > > }
> > > >
> > > > /* Try to append data to the end of skb. */
> > >
> > >
> > > Are you sure TCP_SKB_CB(skb)->when needs to be set ?
> >
> > It's used in tcp_rearm_rto() for calculating a retransmit timeout.
> > ...
> > const u32 rto_time_stamp = TCP_SKB_CB(skb)->when + rto;
> > s32 delta = (s32)(rto_time_stamp - tcp_time_stamp);
> > ...
> >
> > "when" is used as a start point, so I think it's acceptable here.
> >
> > I will add a comment. Thanks.
>
> tcp_rearm_rto() does the following :
>
>
> if (!tp->packets_out) {
> inet_csk_clear_xmit_timer(sk, ICSK_TIME_RETRANS);
> } else {
> u32 rto = inet_csk(sk)->icsk_rto;
> /* Offset the time elapsed after installing regular RTO */
> if (icsk->icsk_pending == ICSK_TIME_EARLY_RETRANS ||
> icsk->icsk_pending == ICSK_TIME_LOSS_PROBE) {
> struct sk_buff *skb = tcp_write_queue_head(sk);
> const u32 rto_time_stamp = TCP_SKB_CB(skb)->when +
> rto;
>
> This means that : at least one packet was transmitted (packets_out is not 0)
This one packet may be repaired:
static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
int push_one, gfp_t gfp)
...
while ((skb = tcp_send_head(sk))) {
...
if (unlikely(tp->repair) && tp->repair_queue == TCP_SEND_QUEUE)
goto repair; /* Skip network transmission */
...
TCP_SKB_CB(skb)->when = tcp_time_stamp;
...
repair:
/* Advance the send_head. This one is sent out.
* This call will increment packets_out.
*/
tcp_event_new_data_sent(sk, skb)
tp->packets_out += tcp_skb_pcount(skb);
}
Looks like we need move setting of "when" in tcp_write_xmit, because it
is set here for all normal skb-s.
>
> Since we timestamp all packets we transmit (look at tcp_transmit_skb()
> callers, all doing :
>
> TCP_SKB_CB(skb)->when = tcp_time_stamp;
>
> Then, write queue head was timestamped properly at the time packet was sent,
> not at the time tcp repair code reinjected skbs into the write queue.
>
>
>
>
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