netdev - Re: [RFC PATCH v6 net-next 3/4] tcp: add in_flight to tcp_skb

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Date:	Thu, 27 Aug 2015 15:54:44 -0700
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Lawrence Brakmo <brakmo@...com>
Cc:	netdev <netdev@...r.kernel.org>, Kernel Team <Kernel-team@...com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Kenneth Klette Jonassen <kennetkl@....uio.no>
Subject: Re: [RFC PATCH v6 net-next 3/4] tcp: add in_flight to tcp_skb_cb

On Thu, Aug 27, 2015 at 3:44 PM, Lawrence Brakmo <brakmo@...com> wrote:
> Yuchung, thank you for reviewing these patches. Response inline below.
>
> On 8/27/15, 3:00 PM, "Yuchung Cheng" <ycheng@...gle.com> wrote:
>
>>On Tue, Aug 25, 2015 at 4:33 PM, Lawrence Brakmo <brakmo@...com> wrote:
>>> Add in_flight (bytes in flight when packet was sent) field
>>> to tx component of tcp_skb_cb and make it available to
>>> congestion modules' pkts_acked() function through the
>>> ack_sample function argument.
>>>
>>> Signed-off-by: Lawrence Brakmo <brakmo@...com>
Acked-by: Yuchung Cheng <ycheng@...gle.com>

>>> ---
>>>  include/net/tcp.h     | 2 ++
>>>  net/ipv4/tcp_input.c  | 5 ++++-
>>>  net/ipv4/tcp_output.c | 4 +++-
>>>  3 files changed, 9 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/include/net/tcp.h b/include/net/tcp.h
>>> index a086a98..cdd93e5 100644
>>> --- a/include/net/tcp.h
>>> +++ b/include/net/tcp.h
>>> @@ -757,6 +757,7 @@ struct tcp_skb_cb {
>>>         union {
>>>                 struct {
>>>                         /* There is space for up to 20 bytes */
>>> +                       __u32 in_flight;/* Bytes in flight when packet
>>>sent */
>>>                 } tx;   /* only used for outgoing skbs */
>>>                 union {
>>>                         struct inet_skb_parm    h4;
>>> @@ -842,6 +843,7 @@ union tcp_cc_info;
>>>  struct ack_sample {
>>>         u32 pkts_acked;
>>>         s32 rtt_us;
>>> +       u32 in_flight;
>>>  };
>>>
>>>  struct tcp_congestion_ops {
>>> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
>>> index f506a0a..338e6bb 100644
>>> --- a/net/ipv4/tcp_input.c
>>> +++ b/net/ipv4/tcp_input.c
>>> @@ -3069,6 +3069,7 @@ static int tcp_clean_rtx_queue(struct sock *sk,
>>>int prior_fackets,
>>>         long ca_rtt_us = -1L;
>>>         struct sk_buff *skb;
>>>         u32 pkts_acked = 0;
>>> +       u32 last_in_flight = 0;
>>>         bool rtt_update;
>>>         int flag = 0;
>>>
>>> @@ -3108,6 +3109,7 @@ static int tcp_clean_rtx_queue(struct sock *sk,
>>>int prior_fackets,
>>>                         if (!first_ackt.v64)
>>>                                 first_ackt = last_ackt;
>>>
>>> +                       last_in_flight = TCP_SKB_CB(skb)->tx.in_flight;
>>>                         reord = min(pkts_acked, reord);
>>>                         if (!after(scb->end_seq, tp->high_seq))
>>>                                 flag |= FLAG_ORIG_SACK_ACKED;
>>> @@ -3197,7 +3199,8 @@ static int tcp_clean_rtx_queue(struct sock *sk,
>>>int prior_fackets,
>>>         }
>>>
>>>         if (icsk->icsk_ca_ops->pkts_acked) {
>>> -               struct ack_sample sample = {pkts_acked, ca_rtt_us};
>>> +               struct ack_sample sample = {pkts_acked, ca_rtt_us,
>>> +                                           last_in_flight};
>>>
>>>                 icsk->icsk_ca_ops->pkts_acked(sk, &sample);
>>>         }
>>> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
>>> index 444ab5b..244d201 100644
>>> --- a/net/ipv4/tcp_output.c
>>> +++ b/net/ipv4/tcp_output.c
>>> @@ -920,9 +920,12 @@ static int tcp_transmit_skb(struct sock *sk,
>>>struct sk_buff *skb, int clone_it,
>>>         int err;
>>>
>>>         BUG_ON(!skb || !tcp_skb_pcount(skb));
>>> +       tp = tcp_sk(sk);
>>>
>>>         if (clone_it) {
>>>                 skb_mstamp_get(&skb->skb_mstamp);
>>> +               TCP_SKB_CB(skb)->tx.in_flight = TCP_SKB_CB(skb)->end_seq
>>> +                       - tp->snd_una;
>>what if skb is a retransmitted packet? e.g. the first retransmission
>>in fast recovery would always record an inflight of 1 packet?
>
> Yes.
> This does not affect NV for 2 reasons: 1) NV does not use ACKs when
> ca_state is not Open or Disorder to determine congestion state, 2) even if
> we used it, the small inflight means that the computed throughput will be
> small so it will not cause a non-congestion signal, but will not cause a
> congestion signal either because NV needs many (~60) measurements before
> determining there is congestion.
>
> However, other consumers may prefer a different value. From a congestion
> avoidance perspective, it is unclear we will be able to compute an
> accurate throughput when retransmitting, so we may as well give a lower
> bound.
I see. Then this is OK for now since only NV uses it. We can enhance
and track tput even during other CA states later. Would that be a
useful feature for NV as well?

>
> What do you think?
>
>>
>>>
>>>                 if (unlikely(skb_cloned(skb)))
>>>                         skb = pskb_copy(skb, gfp_mask);
>>> @@ -933,7 +936,6 @@ static int tcp_transmit_skb(struct sock *sk, struct
>>>sk_buff *skb, int clone_it,
>>>         }
>>>
>>>         inet = inet_sk(sk);
>>> -       tp = tcp_sk(sk);
>>>         tcb = TCP_SKB_CB(skb);
>>>         memset(&opts, 0, sizeof(opts));
>>>
>>> --
>>> 1.8.1
>>>
>
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